Hey Clint, I'll give you my 2 cents worth.
I always suggest halogens for three reasons. (1) They're available just about everywhere from hardware stores to wallyworld. (2) they're pretty rugged. I've broken my share, but all in all they will take quite a bit of abuse. (especially going down the road) Some other types of lights will break from the vibration riding on the trailer. (3) They're fairly inexpensive. I have seen fixtures as low as $6. If you want the good bulbs, that will run you another $10 or so per light. IMO well worth it.
Two other things is that they are instant on. (no waiting for a ballast to warm up or cool down) and they are light weight. (ballasts are heavy)
The down side of halogens...they suck a lot of juice compared to other types of lights. Still IMO the upside out weighs the down.
There is a flourescent fixture that looks real promising to me. I bought one but have not tried it on the boat yet. I plugged it in at the house and was impressed with the amount of light it put out. I fear the the bulb may not withstand trailoring vibration though. If you want more info pm me your phone #. (tooooo much to type.
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I'll let the folks running HPS and Metal halides give their impressions. There are quite a few folks using them these days.
Good luck to ya, hope this was a little helpful.