It all depends.
If you buy usb1 or 2 drive it will be slow (about 28mp/s top speed regardless of drive).
USB3 will be faster but you will need a usb3 port in your computer. About 50 -100mb/s+ depending on the speed of the drive inside.
Firewire is used typically for video work thanks to its more constant speed which it was designed for. expect 40-50mb/s transfer speeds and you will need a matching firewire port to run it.
Esata will use the full speed of the drive so a 7200rpm drive would be faster than a 5400rpm drive. Again 50-100mb/s+ depending on the drive inside. You will need an esata port in your pc to run that.
Now I dont recommend doing that at all. External drives are for backups. If you need more storage add another internal drive or in the case of a laptop, get a bigger internal drive.