I agree with others that say that a 750 is easy to handle, but it does take more effort to take a corner on than a 550 would. I am not saying that the 750 isn't a great handling bike (I wouldn't have gotten one if it wasn't), but if you are looking at which is more nimble than the two I would say the 550. They both have adequate power and handling for the street, you just have to ride them differently to get the most out of them. It's not like we are talking about a 250 here. Most people that ride motorcycles don't even begin to tap into the bikes potential (including me) so in reality you may want more power, but I don't think you really outgrow either, you'll just have to get up on the revs more on the 550 and will have a lower top speed. If I remember correctly, the 550 is around 70 lbs lighter than the 750 and on top of that I think others were talking about the COG being lower as well which both have major impacts on handling. So back to my last post, if you want handling go with the 550 and if you want power go with the 750. It's not that both bikes aren't both adequate in each category, its that you can't go past the fact that one is better than the other in each category when comparing the two. Choose whatever is more important to you: handling or power. For what the OP wants, either would be a great bike.
*I know that the CB 750 handles better than a bike that's bigger than it, but that doesn't mean that a 550 can't handle better than a 750
CB 550 top speed: 102 MPH
CB 750 top speed: 125 MPH
I don't think that most people reach either of those speeds very often so it's more about grunt vs. handling IMHO