We're staying at POR and apparently it is best to prebook but you can book on site. It costs $35 and is a lovely way to view the resorts as it covers both resorts although I think it starts in POR.
I've done one at Fort Wilderness (usually I do the wagon ride, but we did the carriage ride a couple of years ago). The rides are nice, and I would pre-book, even a day earlier (and earlier if you know when you want to ride).
we prebooked ours, did it a couple of nights before our wedding, it was a really nice relaxing romantic way to spend the night. i'd do it again, maybe next time we go
is one better than the other: Fort Wilderness v Port Orleans? I think they're the same price and time and we're staying at neither resort, so would travel for it anyway.