Some tech trees offer 'upgrades' that serve the same function and are an improvement in every way to their predecessor -- a strictly dominant strategy, and therefore it's not a choice, it's a no-brainer.

In some games (Endless Space:Disharmony, for example) new technology is hard-coded to imitate such a forced decision, even though the tech is only a weakly dominant technology. There are times when a price, time or space advantage could have been better, but the interface is designed to prevent such considerations.

If we consider the environment, (such as Galactic Civilization 2's rock/paper/scissors weapon & defense systems) then it can become an interesting interplay. Guns aren't automatically inferior to Lasers, it's a matter of what the other player is doing.

There's also a mentally compartmentallised problem with price. 'Upgrade' techs almost always have a higher resource cost, be it materials, time, space, research points, whatever. But it's not a tradeoff, because that implies a set of valid choices, which is not the case when there's a strictly dominant strategy. So the game ends up senselessly inflating the price of stuff.

To me, a dominant strategy (in the sense of a specific technology being a must-have in order to avoid losing) is a symptom of poor design. Fortunately, it's also pretty rare in my experience.

I think that your interpretation of dominance is overly narrow. If we're talking about lasers always being better than projectiles, then lasers dominate projectiles because there's never a reason to choose the inferior technology. But the strategy space in a game is far larger than choosing Tech A or Tech B. In Galactic Civilizations II, Death Rays are vastly dominant over all other weapon technology in terms of performance, and by the time you can get them cost is mostly irrelevant. But games rarely get to the point that you could research Death Rays without either losing to another player or explicitly passing on other opportunities to win in other ways.

It's generally not about Tech A vs. Tech B (though that choice exists as well), it's about advancing to the next tech level OR expanding to a new city OR building up a fleet OR increasing your espionage abilities or any number of other things. Tech-turtling may well lock you into Tech A vs. Tech B scenarios, but few games lock the player into a tech-turtle play style in the first place.

Some tech trees offer 'upgrades' that serve the same function and are an improvement in every way to their predecessor -- a strictly dominant strategy, and therefore it's not a choice, it's a no-brainer.

From my standpoint, much of the real problem in this lack of choice comes down to the fact that the vast majority of games don't really apply the concepts of scarcity well. As Khaiy said "by the time you can get [the best weapon] cost is mostly irrelevant."

A lot of games don't scale the costs of maintaiing a vast empire as quickly as the profits reaped from having one, so anybody reaching a late-stage game is going to be rolling in resources. One of the primary worries in using "new" technology is the real world IS cost: economies of scale have not yet been developed for it's production and thus it is far more expensive than the "next step down" despite likely being only a marginal improvement. In a different case, for thing like infantry weapons and equipment, small costs balloon quickly due to the sheer volume needed.

In the real world scarcity can be fairly easily measured due to monetary value. You aren't dealing with discrete piles of resources, you're dealing with a certain cost-per-unit in order to obtain something. Because everything can be boiled down to money, you have a real tradeoff - you have lots of other things to use money for. Buying more expensive rifes for your troops, or better missiles for your battleships, means you have less money to use everywhere else. Games generally do a mediocre-to-poor job of representing this opportunity cost. Using my best resource on the military doesn't matter to the rest of the game, because only the military uses that resource.

Another little beef I have with a lot of strategy games is a lack of inequality in efficiency. To take the already used example of Galactic Civilizations (which I personally did not like a great deal), the armor of any particular technology basically cancels out the same level weapon of that tech. I have level two lasers and the other guy has level two shields, I can't do crap to hurt his ships - his shields practically nullify my weapons. This is all kinds of wrong. This means that I can't rely on lots of lower-gunned weapons to overwhelm the other guy with numbers. His defensive tech renders my offensive tech useless, which - historically - is an extremley rare occurence. In order to compete at all, I have to catch up or surpass the opponent in the tech game, thus removing all my other choices for dominance.

...it's about advancing to the next tech level OR expanding to a new city OR building up a fleet OR increasing your espionage abilities or any number of other things.

...Because everything can be boiled down to money, you have a real tradeoff - you have lots of other things to use money for. Buying more expensive rifes for your troops, or better missiles for your battleships, means you have less money to use everywhere else...[But] Games generally do a mediocre-to-poor job of representing this opportunity cost.

...it's about advancing to the next tech level OR expanding to a new city OR building up a fleet OR increasing your espionage abilities or any number of other things.

...Because everything can be boiled down to money, you have a real tradeoff - you have lots of other things to use money for. Buying more expensive rifes for your troops, or better missiles for your battleships, means you have less money to use everywhere else...[But] Games generally do a mediocre-to-poor job of representing this opportunity cost.

These two observations about opportunity cost make a convincing argument against the use of Research Points or any other separate and exclusive technology currency.

Games like Civilization V and Space Empires IV use Beakers/Research Points, which removes the opportunity cost. Normal resources that are spent on expanding a city, building a fleet, increasing espianoge, etc. cannot be used to purchase tech research. But in games like Starcraft and Command & Conquer the player spends currency that is used to build units and buildings in order to develop new technology.

I haven't played either of the games you mention, but in most 4x games isn't there a direct relationship between research points and income? I remember in Civ IV you allocated some portion of your income to research, meaning you couldn't spend it on anything else. Also, you can invest your cash in research-producing buildings, but that investment then can't be spent on anything else, plus you have to pay maintenance on the building. So the opportunity cost is still there, it's just not a "point of sale" kind of investment, like in the RTS games you mentioned.

in most 4x games isn't there a direct relationship between research points and income?

I've definitely played some games where this is true. Don't remember the case for Civ IV offhand, but I'm pretty sure that all the civ games have had a discrete research resource which was gathered from tiles just like anything else. I think you can also invest in research on top of that amount, though. In most of them you can eventually convert production to gold or research directly in the cities, if you don't need to build anything else.