Personally, I think the 9-18 is sharper... but lighter weight and slower than the 11-22.
The extra speed of the 11-22 can be really useful - as can the extra wide fov of the 9-18.
I suppose that, if I could only have one of them it would be the 9-18 - as my subject of preferance is landscapes, and I prefer the amazingly compact lens (it balances extremely well on the G1)
Here's an image with the 11-22... after it had been completely submerged in a river earlier the same evening (luckily mounted to one of my E-3's - so easily dried off with my t-shirt).
E-3 + ZD 11-22mm f2.8-3.5
1/8s f/8.0 at 11.0mm iso100
And here's an image with the 9-18, wonderful lens!
E-520 + ZD 9-18mm f4-5.6
1/400s f/8.0 at 14.0mm iso100
And just for fun, here's a shot with the ultimate 4/3rds UWA - the ZD 7-14mm f4... my son, using the ZD 9-18
E-3 + ZD 7-14mm f4
1/1000s f/4.0 at 7.0mm iso100
Kind Regards
Brian