Like anything else, depends on what features you want and how much you want to pay. Don't know that there is any one computer that is better for tri bikes. A lot of folks on here like the Cateye Astrale 8, a wired computer with cadence, but I couldn't see running the wires on the outside of my aerodynamic, internally-routed-cables, frame, so I got a Cateye wireless w/o cadence. The wireless w/cadence is ~$125 and intrferes with HRMs.
You do have to get a little gadget to mount your computer on your aerobars:
Thanks for the advice on the mounting gadget. It arrived today. Until it arrived today, I have been using my computer mounted sideways...kind of hard to read!