I just bought my wetsuit from my LKS (local kayak shop). I knew how it felt to swim in that particular suit, cuz I had used one in a self-rescue class through the shop. Purchasing the suit ($100) cost less than renting it a few times ($15 a pop) It is a kayaking specific suit: non-slip butt, padded knees, front and ankle zips, low neck.

But let's be honest here- I kayak more than I tri and I don't have a lot of money. (and what I do have I'd rather spend on my commuter-mobile )

If I hadn't already known how the suit worked for me, I doubt I'd have bought it. It's hard to know how a wetsuit really fits until you get it into the water. (and I gave away my scuba wetsuit long ago, so I really did need to buy a new one)

Rent a suit or two until you find one you love. Then consider buying a new one of the same make. If you end up a size smaller by the next tri, good for you! Sell it on EBay or donate it to your local Team Survivor, and get the next size down.

A good wetsuit is a good thing to have. Like a pair of really good hiking boots. They open up opportunities. (Hike that mountain? Sure! Swim in that cold lake? Sure!)