Well, as long as this thread has been resurrected, I'll throw in my two cents:
Alternity is my favorite rpg system of all. I own everything ever published for it, though I don't get to play or run it anywhere near as much as I'd like.
Is it more complex? I suppose it depends on how you think of complexity. At its core, Alternity's mechanic could be viewed as more complex than d20, but Alternity doesn't layer on d20's myriad feat adjustments. So, in play, particularly as characters develop, Alternity remains much simpler to keep track of than d20, I think.
One aspect not mentioned much in earlier posts is Alternity's settings. I absolutely love Star Drive, Alt's main space opera setting. It's gritty, highly political, great for trading and smugglers, filled with tons of unexplored or only slightly explored places, and has a very diverse range of populated locales.
Unlike many settings, it doesn't hang its hat on one particular gimmick or metaplot; though there is a supplement called "The Externals" which gives something of a metaplot to the setting via an alien invasion. I've only used bits and pieces of that.
For the uninitiated, check out
www.alternityrpg.net for TONS of phenomenal fan material on all aspects and setting of Alternity, and check out
www.tequilastarrise.net for massive details on the Star Drive setting.
Carl