I prefer internal drives, because they're a lot cheaper. If you get an external hard drive you don't know what's inside. Is it a 7200 RPM, 5400 RPM, or 4200 RPM drive.
I'm surprised I haven't seen any 2.5" 7200 RPM SATA hard drives locally. 320GB 2.5" 7200 RPM hard drives have been around for a while. 2.5" 500GB hard drives are out now, but I'm waiting for a 2.5" 500GB 7200 RPM drive to be released before I upgrade my laptop.
If you have the money to spare, I'd suggest you get a 2.5" 7200 RPM SATA drive, even if your using it for a desktop. It will have the same performance as a 3.5" drive, will emit less heat, and use less energy.
3.5" SATA enclosure's can be expensive. A cheap alternative would be to get a SATA to USB cable from CD-R King. I got it for 450 pesos, because my mobo's sata ports where filled. They also have a IDE to USB cable for 220 pesos.
SATA to USB
http://cdrking.com/local/products/index.php?action=&temp=2&typeno=8176890-373780-839110596-1439562&prod=IDE/SATA%20Cables&prodcode=3341379-246948-410382915-8619755IDE to USB
http://cdrking.com/local/products/index.php?action=submnu&temp=2&typeno=8176890-373780-839110596-1439562&prod=IDE/SATA%20Cables&prodcode=6700461-804855-990326070-9149949_______________________________________________________________________________________________
500GB Price Comparison Example:
from PC Gilmore (October 29, 2008 Price)
Internal 500GB 3.5" 7200 RPM SATA Hard Disk
Samsung (3,500 pesos)
Seagate (3,600 pesos)
External 500GB
Western Digital MyBook (4,800 pesos)
Conclusion: Price difference is 1,200-1,300. Seems fare considering prices of external enclosure's, but I don't recommend you getting a 3.5" external drive. Do yourself a favor and read amazon.com reviews for the external drive you want to get before purchasing it. If you really want an external drive get a 2.5" external drive that doesn't require it to be plugged into an AC outlet.