I should join on the 737. You will have a standard contract, no loan, proper salary, and neglible bond. However, if you want to live near a particular base which is currently, or planned to be served by the Airbus then perhaps this will influence your choice.
You can always apply for a base transfer to an Airbus base, but then you will be bonded for three years. This is hardly going to be a financial penalty, though, unless you want to run off with your new rating!
I don't see why you would have to wait three years to convert since you shouldn't be bonded if you're entering with the 737 rating.
Not sure about command prospects for 737 FOs, but it's unlikely that they would be discriminated against. But, could be wrong...
FQ