Main thing I'd think about would be the overall configuration. Tri bike geometry is much more geared towards riding down in the aerobars and puts more emphasis on the hamstrings, whereas road bikes are a bit more comfortable when riding upright. I personally have difficulty on tri bikes, mostly because of my build and relatively large upper body- just doesn't feel right to be bent at that angle for the entire ride.
Some road races don't allow aerobars, which is why it's easy if you have a road bike to just use the clip ons... but don't let that make your decision. Try them both out, see what's more comfortable.
Personally, for training a road bike is much more comfortable- easier to adjust position, easier on climbs, more versatile... If I bought a tri bike, I'd ride it for races and maybe 1/4 of my training.