hmmm. I'd say both, though GC0 is more of a challenge even at times where GC1 was still a challenge.
GC0:
Pros: Traps and trap specials, anger, different battle modes, structures on the battlefield. I would like to be able to anger the arcane guardian tho, once at least
Cons: Hard to learn what do skills alter (except for masteries), uniform skill points cost (yep, some skills deserve tweaks in here), high dependancy on orange (no color should prevail, or else the game gets skewed to one side).
GC1:
Pros: Original (it's a pro, since without GC1 there would be no GC0), color balance, pure levels.
Cons: Glowing frame score conception, which does not block much (well, secret levels are something to block, that's for sure,I think there should be something more in there...), gets boring after level 40, "number" skill spoiling the start of every level, useless "mana over time" skill, wrong calculation of "grade 7 or higher" (they fixed this one in GC0).
In short - the GC1 is better while it still provides a challenge, GC0 is better while you get the knack of it, or at least get to the point where you can play aggressively.
(Thystonius: win all levels in a segment between arcane guardians in "heroic" mode to open secret levels in that segment)