In my opinion, Xperia V has a better camera, display and it's water resistant.
Xperia SP has a slightly bigger (not so good) display, so-so build quality (my on/off button has broke twice), mediocre camera, but great looks and a good processing power and MIGHT get Android 4.4 (I doubt it).
So it's up to you to decide which do you like the most.
I'm going to buy XSP, and I like its screen size it's perfect not small not too big !
I hope I won't get any lags or problems with it, even on the 4.3 ^^ ( because I know some people that don't have any minor or major bug with this update, it's 100% working bug free ! )