Generically speaking, in this context, they are interchangeable. You can tag users, or you can mention users, all same same.
Minor change- Took me a minute to spot this one- Tagging users here is unique and seperate from mentioning a user-
Tagging is done from the top of a thread page, where you can now find a User Tag List, and a Tag User button.
For my own use, I default to subscriptions, so I get emails whenever a thread that I'm in is updated, and I use the notifications in the top right of every page to see if there are any mentions & quotes. I disable quote & mention emails (only, everything else is left on) in the UserCP so I don't get multiple emails about the same thing.
FWIW, I also use the Feed function to get all the new posts (not every post, just first post) as a feed in Google Reader.
The combination keeps me up on my threads of interest, and a fast scan of threads as they come in when I have time to review.
Adding mentions & quotes makes it really easy to use the forum!