Ideally, according to the original Dolby design info I read (way back when) the specification was for all 5 ear level speakers to be the same in a 5.1 system. (And the few systems I've heard with the same speakers for surrounds do sound a bit better.) The marketplace pushed back as 90% of the content comes out of the Front LCR speakers and only 10% from the surrounds. (So why would a consumer pay so much for the surrounds was their logic. It made a good 5.1 system expensive for most folks.) Speaker OEMs, in an effort to sell home theater systems with 5 speakers, rather than nothing at all, started to offer "Timbre" matched surround speakers (almost always bookshelves) at a substantial discount over the Fronts. Those few with Thicker Wallets, still went for Full Size Surrounds => pending the WAF of course!
Explaining why a Floor Standing speaker sounds better than a bookshelf sounds - especially when a Sub handles the bottom end in both cases, has been pursued at this Forum and many other ad infinitum. IMHO the difference is getting less and less as Bookshelves (and Subs) get better and better. But I bet one of the Gents here will chime in. Perhaps
@Pogre will, as I think I remember he had SVS Ultras Bookshelves for Fronts for a while and went to their Floor Standing versions. He had multiple SVS Subs too as I remember. Even though the shift to Floor Standing was noticeable, I remember it wasn't overtly significant.
Folks chase small incremental improvements in their systems. But it usually comes at a high price after you reach a "base" level. Some say it's beyond logarithmic. (Tiny increase in sound quality => Huge increase in $$) But in your case a small improvement in your surrounds is almost a wash => pending the WAF of course! (Another plus for Floor Standing is they are less likely to get tipped over than Bookshelves on Stands. A downside to buying used is usually the warranty is not transferrable. But with quality speakers, there's not much risk there if they operate fine when purchased.)
I'd take a look at the used Q750s and if you find them acceptable, negotiate the Seller down to a wash with the Q150s.