DOOM3 will be suX0rs, HL2 will ownz3d j00 (and j00r m0th3r)
With that out of the way, Gabe's statement is at best, optimistic, and more likely just diplomatic. I'll probably buy and play both but some people won't be able to afford both. Just like movie releases are sometimes delayed so as to avoid going up against the latest big blockbuster..
Maybe the timing is just coincidence that both teams were ready for their release in Sep. but it would be nice if the gaming scene wasn't like; years of crappy games, followed by a logjam of great games. Oh well, better than no great games at all I guess.