Do you mean "internal" (for Windows or second drive) or "external" as in USB, Firewire or SATA via a hard drive in an external case?
3.5" drives are cheaper and faster than 2.5" hard drives. If you are looking for a secondary hard drive the Western Digital 1TB Green 3.5" drive is excellent. I own one internally and my second one is mounted in a BlacX Thermaltake via a SATA connection.
I saw the Thermaltake BlacX on sale at NCIX recently.
2.5" drives are roughly 2x the price as 3.5" per Gigabyte but they are nice to save space if you intend to travel out of the house with it. You can get a nice 320GB or 500GB drive for pretty cheap.
If you plan to get an external case I recommend you buy your own hard drive (WD) and look at reviews for a case and buy it separately. USB cases are cheaper than SATA. You need an external SATA or eSATA connector on your case to use SATA. USB is obvious.
2.5" drives can use the USB bus power instead of an external AC adapter.
SATA vs USB:
My BlacX is both SATA and USB. My hard drive just sits in it which I like but this is not ideal if you travel (It is great if you swap hard drives but it also means heat isn't trapped in a case).
Now, I use SATA because I often use the hard drive with my computer (If I don't use it for a while I turn off the BlacX). I have a SATA (not eSATA) connection on my case. The two differences are POWER and HOT SWAPPABLE. The BlacX case has it's own power supply so this isn't an issue, though I had to buy a SATA to eSATA cable as I couldn't use the included eSATA->eSATA connector. With eSATA the hard drive can be connected/turned on with Windows already running just like USB. There is a workaround for SATA; you enter the Device Manager and "Check for New Hardware" (or whatever it's called. This detects my hard drive.
SATA vs USB speed:
A bit tricky. The USB2 interface which most people have is limited to 60MB/second. However, most external SATA/PATA->USB converters max out at 32MB/second. This means that most hard drives will simply operate at a MAXIMUM of 32MB/second at all time. But wait, there's MORE! Due to seek times, the speed of transfer (same as inside your PC) is limited based on file size. The smaller the file size, the longer it takes.
a) For SATA, I can use up to the maximum my hard drive can transfer at. For large files (maybe 10MB or greater?) I can get more than 50MB/second (depends how full hard drive is. A 1TB drive gets maybe 110MB/second when not full and 55MB/second when nearly full for larger files.
b) For SATA or USB with small file sizes like pictures transfers drop below 10MB/second.
c) For USB the maximum (now) is usually 32MB/second. Some cheaper devices drop to a few Megabytes per second (like cheap SD readers).
- For internal storage I recommend WD 1TB Green
- For internal Windows drive I recommend a faster WD drive. Prices are cheap, don't get an older drive. Maybe a WD 640GB. You can Google reviews. Velociraptors are nice but overpriced for most and I'd wait one more year for SSDs.
- For external, get your case and drive separately.
- for external, USB only is slightly cheaper. Get SATA mainly if you leave your drive hooked up all the time.
- *For laptops, if you get an eSATA card your cable might be in the way of your mouse.