I've had both a sled deck and now a 18' enclosed (dual axle steel). There are reasons to have either.
If you can't have a trailer in your neighborhood (or don't have room) a sled deck is great. They make loading your sled and moving around very easy. You can do doughnuts in the parking lot while guys with trailers have to find a way to turn around. I'll probably have to buy chains for really deep snow days for my trailer setup, and I wouldn't need them with my sled deck. Much better mileage with the sled deck. Sleds are harder to fuel on the sled deck and it's no fun getting a downed sled onto the deck.
The enclosed is nice because I have my tools and other gear in there. I also have enclosed storage for my sleds and don't have to have them in my garage. It keeps my truck bed empty so I can always use it to haul whatever. When the sled deck is in the truck it doesn't conveniently come in and out. It takes up all of your bed. The enclosed comes in handy for other stuff too (camping, moving, hauling gear, etc). The enclosed is nice to have a place to change, eat lunch or just get out of the weather.
I'm going with my enclosed for now. If I miss the sled deck too much I can sell the trailer for the price of a new sled deck. If you find a good deal on an enclosed, I'd jump on it. Even if you don't like it used enclosed trailers are easy to sell.