Bob, my two cents, go with timing belt. The efficiency is just as good as v grooves. You will never notice the HP drawback unless you are into the 10K RPM range.
Why I would go with a timing belt? Because if one day you ever decide to play and try your hand at rigid tapping, you know you have the right stuff to keep the spindle in time with the encoder. Not only are timing belts easier to find, they are capable of greater drive ratios then v-groove belts, without the need of a belt tensioner.
P.S. I too have stalled my 5 hp Tree mill, and the timing belt handled it no prob.