One thing I'll add about sabot slugs, is more expensive does not necessarily mean they are better. You need to find a slug that works with the rate of twist in the barrel.

I shoot 2.75" Brenneke KO Sabot slugs and they are awesome on deer. My gun shoots Win. Super X in 2.75" and 3" BRI-type sabots really well too. If I try to shoot a slug with a velocity over 1600 fps, accuracy goes south.

So, just because your gun can take a 3.5" shell does not mean you must use one. I'd stay with 3" shells.