It's all preference, really. But, gfxCardStatus is open-source. (
http://github.com/codykrieger/gfxCardStatus) Transparency is great in situations like these, because that way you know exactly what you're getting. GPUInfoMenu is not open-source, and I believe it uses similar (but not quite the same) techniques to achieve the same-ish results. In recent updates (by my benchmarks) gfxCardStatus has become faster at performing things like the generation of the dependency list, and the GPU switching seems more reliable. It has also been around a little longer (read: a few days). gfxCardStatus also subscribes to a more minimalist philosophy in terms of the icons and amount of information in the Growl notifications.
Plus...I wrote gfxCardStatus, so of course I'd recommend it over everything else.

But like I said, it's all preference.