Like oranges and tangelloes, mostly.
True20 is a "rules lite" 3-class system ... Warrior / Expert / Adept. Things are simplified (ability scores are just their bonus), your character is mostly taking feats every level, and instead of HP you have Damage Saves.
Both are sort of "toolkits" that can be applied to a variety of settings. The new True20 has some setting snippets in the back, all of which are pretty good.
It may be just me, but I think both systems are pretty good. I've used GrimTales since it came out, and I haven't been burning down the house to use True20. For instance, I've been putting together a new scifi setting to play ... I could have put it into any system, but I chose to do a GrimTales-Laced d20Future game instead of True20.
--fje