I've done both and it is a little easier to fly as a trike, because you don't have to worry about nosing over and you have more surface area supporting the plane. On the other hand, if you buy skis, a trike set is a little more expensive.

If you are planning to buy the skis, I highly recommend those produced by Maiden Model Products. You should be able to find them on the web. They are the best I've ever seen and have a really good system for keeping them in place, which is a big problem.