I have ridden both ( an 03 1000, 03 750, and now a 02 750)
The difference between the two bikes is a noticeable increase in top end power and torque for the 1000. At high speeds the 1000 still pulls like a freight train while the 750 still is pulling but not as quickly.
I have confirmed this with a test through Cassiar tunnel that will remain secret.
Another big question is this:
Are you looking at an older GSXR model? i.e an 03 or older?
The reason is that for the K3 750 model, it is still the older generation GSXR with non-radial brakes, and the traditional 1000 frame. The K3 1000 is narrower and comes with superior radially mounted brakes and a 32-bit fuel injection. The new 750's have a very compact frame that is the same as the 600 and are radially mounted with 32bit injection.
I can't say anything about the K5+ but I found the 750 easier to lean at high speeds than the K3. The other thing is that the 750 is much more forgiving if you don't match revs when shifting into a corner. The other thing is wheel spin-since I don't weigh much this was a problem for me if I was too generous with the the throttle.
But given the sheer power and awe-inspiring acceleration of the 1000 any small changes in cornering are completely forgotten.
If you are a highway rider, then a 1000 may be more for you as the 1000 has a counter balance shaft that reduces vibrations. I have found the 750 more "buzzy" on highways.
Overall, if you can afford the 1000 and have the skills to handle this bike, then I would say go for the 1000. I chose to buy another 750 for a number of reasons that are not performance related. On sheer numbers the 1000 is a superior machine
That is not say that you would not enjoy the 750, which in itself is a fantastically balanced and powerful machine, but if you are after sheer adrenaline machine then the 1000 is for you.
Me, I'm happy to have another 750 again