Which GPU Should I get? Help Please

Asus R7 260X is not questionable the better option from the two. I will suggest the Asus R7 260X, or the R7 265 if within budget, that is a much better card and is also a better performer then GTX 750Ti, by quite a bit, though the GTX 750Ti is much more efficient and draws next to no power, which may influence some people.
 
Would you recommend this card then? The Sapphire Radeon R7 265 2GB Video Card, and do you think it will play Next-Gen Games at mid-High settings ( eg Watch Dogs )
 
Yes you can play at mid-high settings but the graphics aren't really next-gen. But expect some patches for optimization for a slight fps increase.
Here is a gpu benchmark of watchdogs, scroll down for the graphs. It is tested with maxed settings:
http://gamegpu.ru/action-/-fps-/-tps/watch-dogs-test-gpu.html
Your power supply should be enough if it has a 6 pin PCIE connector. Can you find out how many amps on the 12V rails. There should be a sticker on the power supply. If there isn't then i would recommend you to upgrade your power supply first.
 
You should get the AMD 7850, look it up on www.ebay.com for a reasonable price range.
The 7850 is way better and it will support future next gen games.