After having both I definitely wouldn't want one with rails. Having open sides makes it so much more useful to me. Of course in my opinion I'd prefer a deckover. No fenders in the way makes it a better equipment hauler.
When I was shopping for a trailer, I wrestled a lot with the decision to go car hauler or landscape utility. I liked the idea of being able to load things from the side with forks, but I also knew the rail would be really handy for hauling certain things. Plus I figured it provided an extra margin of safety when hauling a tractor as well.
Then I discovered the PJ I bought and found my answer. Most rails on landscape utility trailers are welded on, at least the ones that I saw while shopping around. The rail on my PJ trailer is bolted into pockets on the side, meaning I can remove the rails if I need to.
All that said, I've had that trailer about 2.5 years now, and I have yet to remove the rails. However there have been many, many times that I was really happy I had them.
ETA: Here's a couple of examples of why I like having the rails.
They helped keep the hay in place:
I built these walls for hauling firewood and more loose material. Sure if you have a car hauler you can still build walls for this use, but your walls are going to need a little more strength. In my case, the rails are providing the strength, the walls just provide backing to keep stuff from falling through. As a result, the walls are lighter, easier to handle.. I can pop them in and out by myself in about two minutes with no tools.
The day I put them together:
Folded up, ready to put away: