Let the pattern board tell you.
3 or 3 1/2: quite a conundrum. The first factor to the decision is what is your usual hunting situation. This includes the quarry, the range they are shot, and your pellet size preference. Once you have nailed that down, pattern the ammo at that range with 3 in shells, if the pattern density is good for consistent kills, 3 inch ammunition will work for you. If the pattern is too sparse, you will need more pellets. Pattern the 3.5 inch ammo at the same range, if this doesn't solve the pattern density issue, you may have to go to a smaller pellet or get closer to the game.
The second factor is your tolerance to recoil. 3.5 shells have more recoil than 3 inch. Semi-auto shotguns help to mitigate the recoil.
Then it comes down to the actual hunting, are you successful with the ammo you are using now, you are golden. I have a favorite ammo, 3 inch, that I use for geese, it works like magic, geese drop like crazy. So this year my son and I were testing his shotgun with different chokes with his ammo. The results were not as expected, poor splotchy patterns. So, I got out my shotgun, and my high priced ammo, to show my son what a real gun patterns like. The patterns were garbage, over-sized and horrible distribution. But they work like magic on game, now I don't know what to think.
We live in a goose rich area, and most of my son's friends own 3.5 shotguns. And most of them buy 3 inch ammo, it's cheaper and doesn't kick as hard.
IMO the biggest mistake is to choose too large a pellet. And my experience is that #2 steel is as big as you need, smaller may even work better, but larger is not the direction to go, from my experience.
For the last several seasons I've used Kent SilverSteel, my son uses Blackcloud, and another shell that I've heard good reports on is the Score stuff. And then a friend of mine just buys what is cheapest, isn't too concerned about pellet size, mostly price, and he is a very successful harvester.
Also see above post.