Definitely Emirates.
They have been around longer. Have much more experience and exposure. Have stability. Have a better reputation and Emirates on your resume will take you a long way, even if you want to change later on.
Emirates is based in Dubai and thier infrastucture and training programmes are also much much better.
Dubai is definitely the place to be in and Abu Dhabi is just a 2nd alternate.
So make your choice,
Either ways, you should grab it.
Cheers and all the best