Lisa,
Wetsuits~~Long suits provide more buoyancy in the legs. Buoyancy is your friend!

You can rent a wetsuit at
All Tri Sports out on Halsey, and I think they only rent long suits. If you are referring to the arm lengths, then it's a matter of personal preference and comfort in the cold water.
Tri Shorts~~Tri shorts have less padding in the crotch and usually shorter inseams. This can make them more comfortable to run in for several reasons.
1. Less padding means you swim in them and the pad dries out faster during your bike leg.
2. Less padding also means they are more comfortable on the run leg.
3. The shorter inseam is a personal preference, but I'm more comfortable in them than bike length cycling shorts "a huggin' me quads." There are no changing rooms in Triathlons until you get up to Ironman distances, so what you wear needs to be comfortable for all three disciplines. There are also tri-suits that are like having shorts built into a women's tank-swimsuit. They are less restrictive in the waist and they function great on the bike and run, but keep in mind what needs to happen if you have to use the porta-potty during your race. I'm just saying, gals have it a lot tougher than the guys. Most all tri-wear can be worn under your wetsuit, unless of course you're doing the Antarctic Tri. It's tough to get that down jacket and your "down-undies" stuffed in there.

I think the Mock-Tri dates are still up for discussion, but it looks like it will go down on Sunday, May 18th at the earliest? If nothing else, come out and join us for the swim or just to ask a lot of questions. Open water swimming can be very intimidating on race day. I'd advise you to not wait until then to take your fist "dip" in the lake.
C'mon, it'll be fun!!
Doug