FireWire has become outdated now, so USB for sure.
Solid State Drives are more expensive, but are a lot more durable. They are a lot faster, and no moving parts means they are far more reliable than hard disks. They are flash memory chips instead of spinning magnetic platters with read write arm. They are worth the extra expense in my opinion.
For large storage they are just too expensive, and the hard disk type are still perfectly OK. There are a lot of choices, and I find that Otherworld Computers offer great selection and price.www.macsales.com
Alternatives would be Amazon, or local suppliers, or even the local Apple Store.
So it depends on whether you want lots of extra space, or just enough to backup the internal storage of your Mac. Portability is a factor, but most external hard drives don't have separate power supplies these days.
Mac Sales, or OWC as they re known, are a Mac friendly site and are very popular with Mac users.