You'll have a heavy bias toward the road bike on a road bike forum, and you'll see similar bias if you ask on a triforum.
I'm a triguy, so I'd say the TT bike is fine, and could even be recommended, but having just recently got only one TT bike myself to ride (and a fine one - the Cervelo P2C!), I feel that you should answer these questions first:
1) How often do you group ride? If you group ride regularly, the road bike is better. More welcome, and much easier to use in the group due to the integrating shifter/brakes ("brifters"). True TT bikes can be annoying for you to shift in groups, especially in traffic, as the gearshifters are on the barends and brakes on the horns (drops), so you're constantly moving your hands. That said, I've ridden with several groups recently, and have had zero problems with the tri-bike aspect, and can fit into the group ride with no complaints/comments from others.
2) Hills - I didn't expect to be training as much as I am in mountain territory, but I find myself regularly doing weekend climb rides of avg 6-8% inclines, with 10-15% sections. Tribikes are harder to climb with, for sure. If you're going to be doing significant regular climbing, I'd strongly recommend at the least, compact cranks. Even the Cat3 riders in our group ride don't ride the race gearing 53-39/12-25 up those hills, and use either compacts or easier gearing. I'd say the tribike is totally fine for anything 6% or under; it's the long sustained 8%+ sections that will hurt on the tribike due to the geometry and aggressive gearing.
If you're riding mostly moderate hills (not long big mountain climbs 8%+), the TT bike will work fine and will likely be faster in aero. Most of the trifolks I know, both in person and online, mainly race bikes in tris (no crits, cyclocross, etc.) so they absolutely recommended just going with the TT bike, period. Racer roadies tend to do a wider variety of races (crits, etc.) and for those, you absolutely need a road bike, so I understand how they absolutely recommend going road bike. In my opinion, the road bike is definitely more flexible in terms of training options, especially with a compact crank gearing for hills. But then again, go to the triforums, and you'll hear tons of folks who have great road bikes, yet realize that since they're racing in tris, they should be training and racing on a tribike, despite its limitations.
Eventually, you'll probably get both to have best of both worlds. But if you know you'll be primarily tri, go TT unless the two conditions above indicate that roadie would be better.
Last edited by agarose2000; 01-18-09 at 07:09 PM.