Much more humorous, largely because of Richard Dean Anderson's O'Neill character, who was very wry and sardonic about everything. SG-1 started out with a Star Trekkish tone in that each episode was very self-contained and just used the standard underlying plot of "team goes to planet, discovers weirdness, deals with it, returns home." That gave way to much more serialized storytelling in later seasons, although there were always a few self-contained episodes in each season. Atlantis started during SG-1's 8th season and was basically more of the same as SG-1. The writing got pretty stale and, although they kept raising the stakes, they relied on the same deus ex machinas and plot devices to resolve problems. So by the end of SG-1 and Atlantis, everything felt really boring and samey.
SGU is nice in that it brings some novelty back to the franchise. The pilot felt much fresher because everyone was in over their head, there's no super-genius around to fix every problem, they don't have the considerable resources they reverse-engineered from alien stuff over the course of SG-1 and Atlantis, etc. How long they can keep that up is another question entirely, though.