There's another aspect... UltraDMD always splashes a logo on launch, FlexDMD doesn't, so you're ready to flip on the table two seconds sooner.
Thalamus is referring to the problem that UltraDMD will crash out the table when expected to apply commas with country locale settings that specify another character for commas, like a period, like you might encounter in, say, Italy. FlexDMD doesn't have whatever is causing that bug, so it uses the appropriate character and doesn't crash.
There's also promises to add functionality and documentation to FlexDMD, which is being actively developed. UltraDMD has really sparse documentation, and hasn't been updated in several years. Odds are greater than 0 that you might start seeing tables that call specifically for FlexDMD.
My UltraDMD install was working fine, but I tried FlexDMD, and haven't gone back. It's easy to put on over an UltraDMD install, and it's just as easy to take back off.