Hey friend:
Opening statement: I am pump biased. I have no need or desire for a semi-shotty.
Just went through my first shotgun purchasing experience...
Owned a 10-22 before, but I think my gun addiction has began (now that I have the finances), but that is neither here nor there.
Went out duck/goose hunting for the first time last fall with my friends Mossberg 535 in synthetic camo. Gun kicked like a mule with the 3.5" shells but I got lucky or am a good shot and dropped my first duck 5 minutes into legal in low lighting at a ways out. Gun was reliable, could get banged around, never failed on me etc, etc.
I couple of the other lads I was with were shooting 870's in walnut and blue. One of them was a lemon as the pump was chipped and kept catching on the under carriage causing him being unable to pump for his second shot and lots of cursing (lulz for me, misery for him). The other 870 worked flawlessly. Another lad was shooting a BPS in camo and personally (I'm 6' with orang-outang arms and the BPS and 535 fit me perfectly).
Talked to everyone I know, and made a thread about it - its still on the front page as I post this.
I ended up splurging and picked up a used walnut BPS 3" on the exchange here that I am in love with for a really good deal. But if was price was an issue, I'd probably just pick up a Mossberg 500 (as I probably will for a backup/beater in the summer). Personally the 3.5" seems unnecessary, but I'm new and naive to this world, so take that last comment with a grain of salt.
As others have said, get a gun that fits your frame - this is by far the most important feature.
If price is an issue the 500, 535, and 870 are all awesome picks (despite witnessing the lemon). If you have a bit more cheddar, check out the BPS (my own personal pride and joy).
Whatever you do, DON'T rush your purchase. The research, the fitting, the conversations about whats better or worse is almost as fun as the moment you hand over your hard earned cash.
Let us know what you grab and why. I always like hearing peoples reasoning for their purchases.
Cheers.
The dirtiest moustache.