As a former employee of both companies, I may be in a position to offer some guidance. In no particular order:
First, Mountain Air: the deal is a 2 year contract, 23k per year. You are payed a salary, fortnightly, whether you fly or not. You get sick pay and annual leave, payed, and your checks and type ratings are payed for. As I said, its 2 years, bonded for 10k if you leave in the first year, and 5500 in the second, and of course none after 2 years. You will do around 7-800 hours per year, a mixture of VFR and IFR ops, and if your based in Auckland, its all twin time. The Chateau base has an Islander, 206 and 172, and the boys there often come up to AA to help out on long weekends and to cover holidays etc. Accomodation is payed for by the company when this happens. Keith is a good bugger, always keen for a drop, and if you work hard for him, he will help you out when the time comes. Not a "crummy" outfit, and one of the only ones with a fixed contract for the pilots.
Second: GBA, there is no fixed contract, until you get to the senior level, which with the amount of movement going on shouldn't take too long! Not sure of the hourly pay, cant quite remember, but if you do a normal amount of flying per year, it comes out at maybe a bit more than MA, but not by much. But, no fly = no pay. You will work off your type ratings by working on Stat holidays, and not being payed for it, but no cash up front. Checks are done by the company, again no cost. Gerard and Louise are good people, and look after their staff, usually at their place on the Barrier, which is very nice. Not many come from North Shore Aeroclub, and no one I knew swept any hangar floors. You may start on the mighty Cherokee, but you will move onto the twins after a few months, maybe more, maybe less, depending on your time when you arrive. Not sure about annual leave, but the company is associated with IATA, so you can get some cheap holiday deals with the airlines.
In short, both companies are pretty good to work for, they will both get you where you want to go, and both have a pretty good reputation with the regionals. And the guys and girls that you will work with will probably become mates for life, at least, thats what happened to me. I would target both (but don't let the other know!) and soon, summers coming up and its the busy season, so hiring usually starts soon in order to get pilots up to speed when the holidays hit.
Good luck, its a lot of fun, hardwork and an excellent learning opportunity.
Nosey