Re: polish or wax ?
Cant really say which is better because they are for two different purposes. A polish will have an abrasive in it that will bring back the shine if the gelcoat is dull. If its really dull you have to go even more agressive to a "rubbing compound". A wax alone will not bring back the shine if the gelcoat is dull it will just protect it.
As far as marine vs auto I dont think it matters so much, but I do think there is a difference between cheap and expensive whether its marine or auto.