I personally would go for the grey one. Looks bigger. I wonder if either has card slots? Because you sure aren't going to fit a wallet in there. Not my zip around, anyway. Good Luck!!
If it is possible I would go "test" it out in store, put your phone, cards, cash etc. and see if it all fits. If not maybe you can find reviews on store sights or someone here who has it will chime in. Good luck!
I have both in similar design and personally I luv them both because even though they are both small they really serve different purpose specially on outfits and occasion. By the way I have the Prada WOC and MK Hudson WOC