I would think it's mostly a matter of preference.
I don't use online services for the same reason I don't use a kindle - I have an irrational big brother paranoia. But that's just me. Many people feel comfortable using online services, and maybe I'm missing out on something really useful, but well...
I have one flash drive and three external drives of varying sizes. I tend to travel a lot, so I take the flash drive and the most portable of the hard drives wherever I go, and leave the two bigger hard drives with all my backups at my main house.
I'm not sure if brand really means a huge difference in quality as far as external hard drives go, although many people I know will claim it definitely does. I buy whatever comes cheap (but not suspiciously so). As for size, it really depends on your needs.
If you keep your important files on two or three separate places I think you can consider yourself safe enough. At least, that's what I do.
Oh, and sharing your files with friends and family members helps too. Even if you lose all your backups, there's bound to be someone that has kept what you need
