Is there much difference between 70-200 & the 55-750 I already have??
Well not a canon guy so not sure how good the lens you have is as far as sharpness, etc - but my guess is during the day you probably wouldn't see a huge difference. The faster glass could come in really handy for cityscapes at night, or astro photography, that sort of thing.
But if your mostly shooting during the day, well then I'd probably stay with what you have. If you only do the occasional night shot then a good tripod and a longer shutter time could achieve much the same results.