Shelly,
Your assumption is correct, the BUDA shares a common power supply between the two channels. My understanding of the GS-X is the same as yours, that it is a true "dual mono" design and uses a separate power supply for each channel.
Now if you want to talk advantages I'm not sure that I would say that the "dual mono" approach is going to yield as much as some other differences. I don't think there's a bad choice to be made here, each offering some advantages over the other. The GS-X is probably the more brutally pure design, the BUDA offering crossfeed, both offering switchable gain.