Hopefully I can help out a bit, decided to sign up because as I noticed most on here tend to be recommending to step up to the 700's, where as I actually prefer the sound of the 500/100's and budget was not a concern.
I did numerous auditions with the 500's/700's side by side, and while there are gains to stepping up to the 700's, you definitely do losing something as well, and most on here are not disusing that fact.
I am no 'audiofile', so I don't know all the fancy lingo, but i'll try.. The 500's (5.25 mid size) are more 'tight' and 'accurate', and give you more detail. The 700's do 'open up' the sound due to the larger drivers, but they become less detailed, although they do give you a larger sound. This is especially evident when you listen to a live acoustic type recording, and the detail of the instruments. You also have amazing accuracy with the 500's, and this is most noticeable when you listen to just the front right and left, you will swear the centre is on. Same go's for the 700's, just less so. Also very noticeable when you sit 'off axis', not directly in front of the sound stage.
Now if your concern is the bottom end, yes the 700's do give you a little more kick, but not by much, and would recommend a subwoofer with either the 500's or the 700's. I feel like what your sacrificing in the mid-top end by going to the 700's for more bottom end, is simply not worth the sacrifice when you can make up for this with a sub, and keep the detail and accuracy of the 500's.
Then again, this is all based on a smaller/mid type rooms size, I can see how if you have a larger room, that you may want the larger mid size for fill, and that type of sound I described from the 500's may not be what your after when you yourself are listening, and so the 700's may just be what your looking for.. Best thing to do is to A/B test them side by side on the same A/V receiver, and definitely cover all different genres of music while listening. Also want to mention that I made sure they were all auditioned on Yamaha AV's, as this is what I knew I would be running, and I would recommend doing the same.
P.S. - Watch out for lazy salesmen that don't want to swap speakers to a different receiver in the showroom etc., give your money to one that actually cares about you finding your perfect sound, not just an easy sale.
Initially I was biased against the KEF's because of how they look to be honest (shallow I know) but once I closed my eyes and heard them, it's that type of sound that puts a perma-smile on your face, and was sold
I run the Q500's, Q200, Q100's with a Definitive Technology Sub on a Yamaha RX-V1800 and the sound is incredible.
Hope I helped a little as I'm new here : P