Depends on the shaft as well.  If the numbers don't lie, I would trust the rep.

I went to the AD DI 7X and have the 8.5 913 D3.  It is the perfect combination of spin and flight that I want.

TT

I'd go with whatever the Titleist rep fit YOU for. If you have any doubts, don't hesitate to get a hold of him/her again, explain your story, and I'm sure that they'd be more than happy to check things out for you again. 

Do you remember your numbers? Swing speed, ball speed, AOD, etc.? Depending on your swing/ball speed, that launch angle may be a little high. 

Ball speed was 143

swing speed was 106 

launch 14.2

spin was 2640ish

carry 257....

with the 7.5 head and Oban Devotion 6 stiff (4 flex)

My concern was with future flexibility if I needed extra loft. Would it be better to get the 8.5 to play at 8 and in the future be able to go to 10 if need be. I don't usually keep a driver more than 3-4 years.

Trust your fitter. I have second guessed a fitting before and it turned out bad.

honestly trust was not a concern for me.

I do trust what i was fit in to. But im trying to be proactive to wards the future.

Does a 8.5 head delofted to 8 play dfferently than a 7.5 head lofted to 8?

thats where my confusion lies...

the specs put me in the middle of the two heads

If you go with an 8.5*, the lowest that you could deloft it would be 7.75* and if you choose the 7.5*, you could either adjust the loft higher to 8.25*, or, 9*. a .25* or so might not seem like a lot, but it can make a world of difference with the right shaft, etc.. 

Did you use a Pro V/V1x, or whichever ball you normally use, during your fitting? 

Really, though, if your Titleist rep fit you for a specific driver/shaft, I'd go with what you were fit for. Assuming that he/she tried out various shaft/driver/Sure-Fit (settings) combos. 

If you have anymore questions, don't hesitate to ask. 

maybe a TT employee could tell me if the two heads will play diffferently set at the same loft?