From their pricing page:
"Capacity is defined as the total uncompressed user data found in TokuDB tables on each MySQL server. (Note that the capacity calculation does not include indexes.)"
It's a lot more iOS like. No more app drawer, instead all your applications are icons on the homescreens or in folders on the homescreen. Also there's an iOS like dock at the bottom where you can put up to 5 icons.
I can't really comment on how they get the better performance cause it's closed source. All the ports for the various devices are using the drivers from CyanogenMod I believe so it just has to be better tuning.
MIUI is currently running Android 2.3.5. These guys push out a new build of MIUI every Friday and I highly recommend it over any other ROM. Been running it on my EVO since late last year and never want to go back to stock. There's ports for a bunch of different devices here: http://miui.us/
