Technically, since it is a japanese word, we should use the japanese pronounciation, and that would also mean we would use an uke. But if you pronounce it in the english way, i guess, you would say an uke (because uke sounds like juk)
So, while speaking, you could probably use both, but I would suggest using an as that would be the correct way. Its not like anybody is going to hunt you down for not using it, but meh.