Personally i think you would be better to spend the extra £10 or so and get the Ti, as it will get more performance. How much more i am unsure
What MOST people can expect:
750 = 260khs
750ti = 300khs
I can likely go more on the 750s, but my 750ti's seem to max at about 305-310 now with the newest cudaminer and high overclocks. Previously I was only able to get 288-295 consistently.
1,560khs for a 6x750 rig at about $720 in gpus (ones i got were $120)
1,800khs for a 6x750Ti rig at about $900 in gpus (@$150, mine were $165 on average though, so $990)
**Also keep in mind the 750s are supposed to use less power and I've never tried modifying the bios at all**
I'm hoping to get GTX 760 second hand for around $210 and SLI'd with my gaming rig (I get ~310kh/s with my 760) but the power usage won't decrease much from 5850+6670 combo