I think both islands are great but quite diverse and if you're coming all the way out here and have the option you should really experience both...!!! I'm sure your girlfriend could easily pick up work in Christchurch once you're there if that was a concern... there's so much casual work around so it shouldn't be a problem.
As for getting between the two islands it's pretty easy and not too expensive... you could fly from Christchurch to the North Island (or vice versa) relatively cheaply and then get a bus out to Hawke's Bay, particularly as you would be able to book a few months in advance which would get you cheaper tickets. Maybe have a look at
nakedbus.com ? New Zealand cheap bus fares, city to city bus company NZ for an idea of bus prices... they do tickets from $1, particularly booking so far ahead you should get a good deal or
Pacific Blue Home for flights prices.
Hope that helps and you have a great time whatever you decide
