The 750 Ti is the overall faster card, although they perform really close to each other in most games. The performance gap between it and your current card will depend on the game. In the titles that favors nVidia GPUs like Bioshock Infinite, it will be a rather large performance gap, in the titles that favors AMD GPUs like Company Of Heroes 2 your current card will be faster by a bit of a margin. You can take a look at the benchmarks here:
http://www.anandtech.com/show/7764/the-nvidia-geforce-gtx-750-ti-and-gtx-750-review-maxwell/11 The R7 260x in the charts is a rebranded HD 7790 eg; they are the exact same card with a the R7 260x having more recent features that don't influence performance. So it is all up to you really, whether if you feel like the performance difference in the games that favors the GTX 750Ti is worth it or not.
Oh also I forgot to mention that the 2 GBs VRAM of the GTX 750Ti may come in handy in the future, a lot of new games are using a lot of VRAM and the limitation of VRAM on your current HD 7790 will bottleneck its performance.
An important question though, what is the brand of the GTX 750Ti you were going to swap your card for? Does it come with a power connector? If it does that would be great news, these nVidia Maxwell chips are great overclockers.