what do you feel is more important for better preparing to entering the industry,

[a.] a published title (aka a self-published indie game) ... [that] might not provide the best avenue for improving my own skills, since I would not be working with anyone more experienced than myself.

[b.] or experience in a similar industry? ... As a self-taught programmer, I feel I could learn a great deal interfacing with other developers in even a non game related, production environment, but I'm concerned that the general skill sets that I possess and want to improve, will not cross over between jobs ( 3d math, in engine scripting and UI, and other similar. ...

Typical 2-choices question that ignores options [c] and [d] "All of the above," as well as [e] "None of the above."

The way you've described your option [a] you've painted it as an undesirable choice. Clearly you think is the way to go, but you purposely haven't provided details about that makes a good choice. So I vote for either [c] or [e], in which [c] is "work collaboratively on an indie project, not solo." And if you have to make an income, then do both and [c].

I don’t see why making your own project wouldn’t be enough of a learning experience. Whenever I wanted to learn something new I decided on a project that was motivational to me yet just above my skill level so that I would be forced, if by nothing else than my own motivation, to learn and grow.

The overall process was described here.

Working solo can’t be compared so easily to working with others because it entirely depends on the quality of your choices.

There is no reason you can’t work solo and still learn the 3D math, engine scripting, etc. effectively by asking questions when you get stuck.

You only need to pick a project that actually has all of the elements, or at least some, that you want to learn.

And I would go with learning them one-at-a-time, since if you are working solo you will have too many tasks otherwise, and if you are working with a team you would only have one task while some other guy or girl does the engine scripting. In either case you learned only about 3D math and nothing about scripting. And if you worked with the same team on the next job, your task would be the same and so would theirs. You would never get a chance to round yourself out. Even if you are motivated enough to drop off your 3D knowledge and start from scratch with scripting on the next project, the guy or girl who just did scripting would not be so happy to be unable to carry over any of his past experiences while watching you make all the same mistakes he or she did with the result of a product that takes longer and has more bugs than it should.

Ultimately, I would go for working solo.

You can pick your own projects, you are free to handle any and all of the tasks, and you won’t get stuck with the same task on each following project.

And you are free to have multiple projects going at once. I tend to have one main massive project with a bunch of smaller ones on the side (not at once). The massive ones take many years and each smaller one takes a month or so, and whenever I get tired of one I go to the other. This keeps me learning all the time.

Your mileage may vary, but it is generally hard to get more efficient at learning than that.

L. Spiro