Having auditioned both (however not side to side) I think I can give a pretty opinion here....
I am curious as to which model in the 700 series you are looking at. The 704 is definitely the closest to the 633, and the overall differences are minimal. They both have good SQ with a slight edge in favor of the 704 as it is a bit more accurate, particularly in the low end and midrange. Both are fairly efficient so receiver power should be sufficient, but a separate amplifier will definitely bring out the best in them. I suppose the big question here is do you want to pay $1000 or $2200?
Now, if you want to throw the 703 into the mix....
It is without a doubt the best of the 3. Everything about this speaker is better. The dedicated midrange adds detail, the dual midbass drivers fill in the low-end very accurately. Both of these features are a perfect compliment to the tweeter. However, the price tag on these is around $3000. There is another alternative in the B&W lineup: the CM7 which is, in my opinion, B&W's best bang for the buck at $1800 (I think). I actually own the predecessor to this model, the CM6 and I love them. They are a very efficient speaker, sounding great off of receiver power.
The 633s are still an excellent choice, especially for the price.