Let's see in order of detail there's:
1/144 scale kits, not sure if any are classified as HG
High Grade: HG kits have reasonable articulation, but don't have the internal "skeleton"
Master Grade: Have removable armor and the skeleton, and good articulation (not sure about hand articulation in each finger though...)
Perfect Grade: Hellaciously expensive, but have extreme detail and articulation, moreso than the MG kits.
At my level, the MG kits are more than enough but I aim to get a PG Wing Zero Custom eventually.
If any of my information is incorrect, please feel free to flame/hack me apart as you see fit... I'm no pro, those are just my observations.
EDIT: BTW, NICE job on the kit, looks great!