If you plan on getting corals and/or anemones i would get a T5 HO fixture. They are pretty well priced, and they will allow you to keep just about anything you want. As long as your tank isn't more than 30 inches deep (T5 don't penetrate water as well).
I know you just want to keep soft corals, but from experience you start out wanting one thing, then see another and think man that is cool looking. If you truely do think you will only stick with soft corals then you can get another fixture, but i'm willing to bet that you will want more than just soft corals once you get into things. Go T5 or MH.