Don't think there is a nickels worth of difference in performace/reliability (slight edge for WD). I believe both are single platter which means WD has greater density leading to possible performance gain over Seagate. Also the WD is sligthly cheaper per MB.
As previouely stated stay away from the seagate -11, and for me the verdict is still out on the Seagate -12. Wanted two 500 G -12's), but opted in the end for 2 WDs (blk 640's)
Added: for seagate might want to look at:
It's a -12, is 500 gigs vs WD 320 and is thinner and uses higher density single platted (500g per platter)
AND is only $3 more than the WD
Seagate used to hold their own with WD. However, they've had some issues lately w/ the firmware problem and overrated seek times. WD has been solid. I don't know why anyone would go with a different manufacturer.