I want a really nice experience, no lag, etc.
I would use an external SSD with either.
It was said to get the most ram and processing power for my money so thats what I tried to do. However now I'm looking at the i3s and the money is less and the display is 17" and beyond.
What do you all recommend is the larger screen still much better or is the smaller laptop screen (15.6 ") just what's used now, and just as good for amount viewed?
Honestly, you're going to have to make some compromises at the price-point you seem to be targeting, you can get a pretty decent computer experience out of a $350-$400 laptop these days, but its not going to be the nicest experience.
The machines you linked to have low-resolution displays -- 1366x768, and that's really too low to be very productive with Visual Studio or other productivity tools, and the pixels will be quite chunky on a 15.6 inch screen. Plus, its not screen size alone that gives you usable real-estate, its having a balance of screen size and resolution -- I find that you really want 1920x1080 or better, and that its a good fit for most screen sizes (though, its too many pixels for anything smaller than 13.3 inches, and even that is stretching it) -- 1600x900 is a good resolution for a 13.3" screen too, though I'd say its the bare minimum resolution for on-the-go productivity. Screen quality and viewing angles are also important considerations for your comfort and ergonomics.
You also want an internal SSD. You can get some pretty speedy external SSDs, but they're not inexpensive and you could have an internal SSD for the same price. Try to find a laptop with a 128GB SSD inside or larger; or get one with a mechanical drive that you can easily change yourself without voiding your warranty -- and then buy an SSD and install it yourself -- afterwards, you can put the mechanical drive in an external USB enclosure and use it for extra space and backups. Be aware that if you install your own SSD this way, you'll need to jump through a couple hoops to get your OS on it, but its doable.
In a laptop, screen-size also directly affects portability. A 17" inch screen sounds good at first, but you might not feel that way after lugging the thing around for a day. If its too heavy or bulky that you never want to move the thing, you might as well have built/gotten a desktop instead, since you'll usually get more computer for the same money and have more options to expand and upgrade. If you're going to pay the laptop premium, it needs to be portable in practice, not just portable in theory.