I prefer
Zoey and here's why: The accent in Zoë is hard to type if you don't have a keyboard that works with it and I'm sure computers will be used even more than they are now when a future Zoë is born. Now,
Zoe, I used to know a
Zoe who pronounced it Zo and used Zo-ee as a nn. So, if you're pronouncing it Zo-ee,
Zoey is just all around easier.