3 1/2" mags don't make your **** bigger. If you shoot them well, then good on you, but I haven't seen any reason to use them. I shoot a lot of buckshot for coyotes, it's nice using 3" over 2 3/4" for the shot count, but the 3 1/2's don't make up much more ground in my book, not worth the beating or recovery time for follow up shots should one be needed.
Don't fall prey to the opinion that buckshot gives you leniency for poor shooting. Each pellet of 00 buck is not even quite as powerful as a .22mag, so you need good placement, and ideally need more than one 'golden pellet' doing the job. Pattern your scattergun properly, know your POI and spread at different ranges, and practice shooting moving targets. I use scrap 5gal bucket lids, have a friend roll them across the ground. At 20yrds, you'll quickly see how foolish it is to think that 00buckshot REALLY changes the game on deer over slugs or a rifle.
But 00buck at 20-40yrds, well placed, will kill a deer.