Get whichever you find cheaper. Both are good. Seagate has longer warranties but if a hard disk is going to fail, it will fail in the first month anyway, most of the time.
I did some more research and this Seagate is an older disk belonging to the 7200.10 family. The newer Seagate product is 7200.11 with 250 GB per platter.
Maybe I should wait or get the Western Digital.
I also found another WD drive: Western Digital 500G Caviar SE SATAII 16M Cache Hard Drive - WD5000YS
Not sure what are the difference between WD5000AAKS and WD5000YS.
I think the YS is designed for non-stop use, as in corporate servers. Not sure though...
I've read a review about the 7200.11. There are some improvements over 7200.10 (speed, noise, power consumption), as expected. Nothing spectacular but every bit helps