In my personal experience (I've only had the Gigabeat for 2 days now), the overall feeling is that the Gigabeat port is a solider port for the moment. Because it has a USB mode, you never have to leave Rockbox, and I *believe* the battery life is on average better, though I've not made a personal test yet.
As well, once Rockbox has solid video support, it will likely (but not definitely) be better on the Gigabeat, at least for quite some time.