A mag trainer uses a magnet for resistance. There is a limit on how much resistance you can get out of a mag set up. Fluid uses some combination of fluid and magnets (usually). The addition of the fluid gives you more range of resistance. Over the long term and heavy repeated use, fluid can leak and would likely be repairable. Though I'm not sure how often this happens. May want to check out REI online and your LBS to see what is carried and see if they have tips too.

Trainers are noisy too. And you may want to consider trainer specific tires since regular tires do not hold up well to the heat and friction.