The difference between high velocity steel and standard steel on ducks and geese is night and day. 3.5" shells allow you the pellet count to have an effective pattern with a useable shot size in a high velocity loading at 60 yards rather than 40 like 3". 3.5" HV steel WORKS. As to Daves comment on the guides not being too impressed, I find that very easy to believe. Most guys never pattern their guns, most buy a super dee duper gun and the shells that are on sale at the moment. Most guys shoot poorly on game. I have read that the average shell count per duck is around 8 shells, think about that.
I also agree with Dave that with Hevi and other non-toxic wonder shot there is a lot less need for 3.5", but that only applies if YOU are going to shoot $2 a bang wonder shot. If not, and you are willing to pattern and practice, get the 3.5". My fall is dedicated to waterfowl hunting, the 3.5" made a huge difference for me.