I personally plan on having 2 SD cards, one geared toward single instance apps like emulators and media player. And another with Xfce and all the UMPC type apps installed on it. I really don't see how the Pandora is going to have one GUI to rule all of it's faces. It simply hasn't been done yet on any device before. I've brought all this up before
here. In a nutshell, the fast switch from full X environment to single app environment leaves some questions to answer like what background apps should be left running, what happens when the single app returns, and all manner of other hard to answer questions.
So, unless someone has come up with something new and miraculous. Most people in the emulator/mid camp are going to want a single instance style interface similar to gmenu2x, while the UMPC camp will want X and a window manager of some sort.
When talking about the WM to choose. Ultimately it won't matter, because if yours isn't the one that Team Pandora chooses, you better believe it will be available shortly after launch, because someone will compile it if they can. I'm personally rooting for XFCE, all the joy of Gnome without the bloat. I use it on my lappy, and desktop, and couldn't be happier. However, KDE4 with it's plasmoids sure looks pretty.
Using an SDL or GL based frontend on top of X will be subject to the same types of issues as a full blown X/WM environment, and doesn't help to solve any issues related to using X. While looking nice and customized, doesn't do much for the end user above what a normal WM might do, and would take a considerable amount of programming to bring to the same level of usability of even the simplest of WM.