I have had both the front relief zipper and the drop seat. The drop seat is a little bulky, but I find that I can get it situated comfortably between my backband and my seat so it does not cause any problems in my sea kayaks. In my whitewater boats my outfitting is a little more snug and I find that I am sometimes aware of the zipper against my leg where my hip pads are. I would suggest if possible trying on a dropseat suit and sitting in your boat to see how it feels. If it's comfortable, go for it - it's pretty awesome for long paddles and much more convenient than the front relief zipper. One other thing to consider is how snug your skirt tunnel is, mine is a pretty close fit on my waist, but getting it over the added bulk of the drop seat is a bit of a pain.
The Connyak BBS