To me, there is only one absolute that I would recommend no matter what: A zipper entry that you can open and close yourself. That means, mostly likely, a diagonal zipper across the chest. But, Mobby's does make a suit with a unique zipper that goes around the back that you can do by yourself.
Beyond that, your choices are:
Neoprene, Crushed or compressed neoprene, tri-laminate, or breathable tri-laminate (as a simplified list). They all have their pros and cons.
Attached boots or socks.
Seal types (for wrist and neck). Neoprene, latex, field-replaceable latex, or silicone (which would always imply field-replaceable).
Pockets. One or two. Thigh on the side or on the front.
Pee valve, yes or no. These are easy enough to add later, if you don't get one up front.
All of those things have pros and cons. Only you can assess which ones make the most sense for your budget and your plans for diving.
I had a Bare XCS2 Tech, which was compressed neoprene, diagonal chest zip, latex seals, attached boots, no pockets, and no pee valve. I sold it recently and haven't missed it at all. It was heavy, really heavy when wet, took forever to dry, really hot to have on in warm weather while gearing up and during a surface interval, and seemed to hold a little more of an air bubble than my other suit.
My only drysuit, currently, is a Waterproof D9X. It is breathable tri-lam, diagonal chest zip, field-replaceable silicone seals all around, neoprene socks, thigh pockets, and no pee valve. The seals and socks are mods that I had done after I got the suit. I really like this suit. I use it in the local quarry which gets down to 37F at the bottom and I have used it on technical dives in 80 degree water, and have been comfortable in both extremes and all in between. Being a lightweight tri-lam, it seems to shrink-wrap onto my body better in the water, to have less of an air bubble to manage than what my Bare suit had. And, now with the socks, I can turn it completely inside out to dry if/when I get water inside.
If you have to buy via mail order, having good measurements is key. But, being able to try stuff on would be even better. If you ever down to the DC area, my local shop stocks a lot of drysuits. They have Bare, Waterproof, Hollis, Mobby's, and ScubaPro drysuits in stock. They are also a Fourth Element dealer and have at least one or two of those in stock, but not a range of sizes, so you would just have to get really lucky for them to have one of those in stock to fit you. Likewise, for DUI. They just have 1 or 2 in stock, I think. They don't have all models and all sizes of the other brands, either, but they have a pretty good selection. And generally good prices on all of them.
