The UltraMobile PC version of ArtRage is just a UI change (and minor work-flow changes) to ArtRage 2.2
If you have a screen resolution of 1024 x 768 or larger and have a stylus and tablet pc that support 'hover', I'd recommend you get the standard ArtRage 2.2 Full version.
If you have a mobile PC device around the 800 x 480 screen resolution, and a stylus (or fingers!) that dont support 'hover' (also known as proximity or rollover) then you're better off with the UMPC version.
In terms of functionality ArtRage for UMPC is identical to ArtRage 2.2 Full version.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"