I have the Keeper, and LOVE it. I seem to remember trying either the softcup or something like it, and I hated them. They were hard to handle, and messy. The Keeper is easy to handle, like not wearing anything at all, and can be left in all day (8-10 hours). It also isn't disposable, which is not only better for the environment, but more convenient. You never run out of "supplies" and have to make an emergency run. For some reason, I find that I'm more comfortable using light days liners for the last couple of days of spotting, but for my heaviest days it is fabulous.
I hadn't heard about the Diva until now, so I can't comment, but it sounds like it might even be better. I've been using a cup for about 12 or 13 years now, and I wouldn't go back. In fact I had to go back briefly around the time I had my daughter, and I forgot how awful the standard options are.