I prefer to go with Nvidia due to driver quality. I've always had better luck with Nvidia WHQL drivers. I avoid the beta drivers. However, there are others here who would claim the opposite. That's just my advice.
As far as the raw performance goes, from what I just read on various reviews of both, it looks like the R9 270 performs significantly faster than the GTX 750 Ti. The 750 Ti looks to be in the same class as the R7 265 in performance, and a little slower at that.
I'd still go with the 750 Ti, but that's your choice.