I'd say that External HDDs are more reliable, because it's hard to really damage them (as long as you don't put a big magnet near them, or throw them against the wall, lol).
SSDs aren't very reliable, because any SSD (even the most high-priced one) can die easily if the memory controller fails suddenly, and that happens quite a lot these days, even with the memory controllers that are considered to be the best (Marvell, Indilinx).
If you're lucky, your SSD would work just fine 3+ years without any problems at all...but in the worst case scenario your SSD can die just one week after you've bought it, or even at that same day you'll turn it on for the first time! And that can happen to ALL models of ALL manufacturers (even GODLIKE Intel's, though their SSDs are considered one of the best out there).
I really suggest that you get External HDD, not SSD.
You don't really need ultra-high speeds for external storage, because, DUH, it's EXTERNAL S-T-O-R-A-G-E
, you won't be using it 24/7, you'll only be doing occasional back-ups on it.
OP, if you're looking for advice on what particular HDD manufacturer to choose...I highly recommend getting Seagate's "FreeAgent GoFlex
" line of External HDDs, they are FANTASTIC.
I personally have 5 of these External HDDs, 2TB each (Four "STAC 2000200" and one "STAC 2000202"), been using them for quite a while, and they never failed me even once.