Why not both? I use both of them as my starting duo in my Atletico Madrid save.
If you can only get one, I prefer Phil Jones.
1. positional versatility, he can cover for RB, DMC, and even MC in a pinch.
2. leadership abilities - he averages at least 16 determinatin/16 influence and makes a perfect captain for Chelsea
3. with Phil Jones you don't have to worry about work permit or foreigner restriction
Make sure to unsettle him and you'll be able to sign him for cheaper. The key is making sure he is interested first though.