In terms of what? The pair of 6.5's will have a little more cone area than a single 8 but likely won't be able to play as low. Unless it is due to space constraints, I would stick with a single 8 (ported if you can). There are quite a few choices out there. What kind of car is this going in and what is your goal?
"A man is a success if he gets up in the morning and gets to bed at night, and in between he does what he wants to do." - Bob Dylan
Looks much better (and I'm sure sounds) better than the stock grill. There are guys with the deeper 8W1 driver in.
IMO, you should just make some spacers and go for it because the ZR800's are really nice midbass drivers...then put some 5 1/4" or 4" up top for midrange. Would go that route over a 6 in the bottom and a 6 at the top...would be a shame to take out an OEM 8" and put in a 6"!