I'm using Google App Engine and storing users. Before, I was using MySQL with an auto-incrementing int for my userId field, but now GAE auto generates a key for each new entity I store, such as `g5kZXZ-aGVsbG93b3JsZHIKCxIEVXNlchgNDA`, but they also automatically generate an auto-incrementing ID int too. 

Which one should I use in my client code to store as the userId? Are there any advantages to using the long key that GAE generates, or is using the small int ID the same thing in terms of performance and look ups? Are there any advantages to one over the other, or is there practically no difference?

**Edit:** Sorry my question was not clear enough. Here's what I'm asking:
It's not about length, but does having the lookup key put me a step ahead of not having it? Because if I wanted to look up a user, I'd have to look him up by email, but now that I have the key for that row in the "table", does this give me any sort of advantage?