What are the pros/cons between a system based on Compact Flash and one based on MMC?

From the research I've done, I've concluded that the prices are about the same between the two. The only obvious benefit to MMC would be a smaller size, but the circuit boards are about the same size for each of them so the size of the memory would probably be irrelevant. I'm looking at 4GB and that is available in both CF and MMC too.

From reading the wiki, though, I do recall something about not being able to boot off of MMC. Is this correct? Can you boot off of flash?

Also, I know that there are no SD drivers, but if I went with MMC could I buy an SD card and use it as an MMC card? Nobody really sells MMC anymore, everything is SD (other than from gumstix).

I'm leaning towards CF, though. I know gumstix has switched back to Type-I, does this put any memory size limitations on me? Can I run 4GB? What abou tthe Ultra-X stuff?

Thanks,
Jason Pacheco