Zincs won't help, as electrolosis is not the problem, corrosion (rust) is the problem. If you can afford it, the aluminum is the way to go. If not, you definately need to galvinize (Dip Method) the trailer. No mater what you decide to do, you need to follow a couple of rules if you plan on keeping your trailer for more that 5 years:
1. Freshwater rinse immediately after every dip in saltwater (which means the trailer will get washed twice per boat launch, once when you put the boat in the water, and again when you pull the boat out of the water).
2. Use Salt-Away as often as you can afford to. This is especially true with washing your boat and flushing your engine. I use Salt-Away after every session, on both the trailer and the boat.
3. If you use a galvanized trailer, take care of any new rust immediately! If you see rust forming, use a rust killer like Extend and then use LPS Cold Galvanize after the rust is converted.
Other than that, you should be good to go.