I have four seagate sata hdds .. all are working fine with average working temp of 38-40 degress. good cabinet ventilation does affect hdd working temperatures. I had a problem with my old seagate 200gb hdd ..replaced it thrice ..later found out the main culprit was my psu and faulty sata cables. now since then no problem whatsoever ..seagate after sales service is excellent. its just that u have to carry it to service station.
heard, wd are offering onsite warrantty .. if its true then great. then only go for wd ..performance wise it depends on the model ur getting.