Deciding on a bike

I have been doing some triathlons for the past couple years but have decided to start getting more serious with them.  I have been looking at a couple different bikes and am trying to decide what would be the better pick for my bike.  Currently I am riding a specialized allez and am only able to purchase one bike at the present for both training and competing.  I ride a lot in cities and on hills and don't know how much a dedicated TT bike will work for my training.

I am wondering if anyone has had experience with the aero road bikes.  If so, are they worth looking at and are they anywhere near a viable option for a bike if I set it up with clipon aerobars? or should I just go ahead and buy a TT bike?