Do you need very high performance (probably not, USB doesn't do that)? Is your machine short of memory? Then use UFS. For all other uses, ZFS. It has several important advantages: It protects all data with checksums, it writes to the disk in a pattern that is much less likely to be vulnerable to losing recent data on a crash, and it has scrubbing, which helps find disk errors before they become problems. All these things matter a lot if you have a redundant system (with multiple disks), but even on a single-disk pool they help. The cost of that is: higher CPU and memory consumption, and/or lower IO performance if CPU or memory constrained.
On my server at home, I have an external USB-3 disk that I use for backups (it is on a relatively long cable, and physically installed in a big and pretty fireproof safe). Works great. I tried this years ago with a USB-2 disk, and at that time USB wasn't reliable enough for continuous production, so for a while I was using eSATA, but with USB-3 it has worked perfectly. The disk is a cheap 2.5" Seagate that I bought at Costco (giant warehouse store in the US).
