I'd take either as a CB. but Jones has a few better qualities such as the variation in his positions and also like said before he's homegrown. it's very easy to unsettle zouma though I got him for 15mill as Liverpool
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.
Since you are Chelsea, I would get BOTH. You have easily enough money for it and no use to wait. They are both elite players and will become the core of the team that will make you champion after Terry retires. Talking about Terry. Zouma becomes amazing if you let Terry tutor him. Zouma will become a top 3 defender in the game together with Varane and Papadopoulos.
Phil Jones is great because he can do nearly everything. RB, WBR, DMC, Box-to-Box and DC. Furthermore he will become a great leader if you just give him 4 seasons as co-captain.