I've never owned a tri/tt bike but I've ridden them. Unless you are serious about the sport, I'd recommend a good road bike. You can always get some bars to replace the drops and you can get in a decent aero position with those. I good road bike can pretty much do it all. The only reason I could see going to a tri/tt bike is if you compete in triathalons or time trials.
Of course this is just one person's opinion. I'm sure you'll get answers all across the board.