Did you buy the videos, or record them from tv?
I would get the dvds for two reasons: (The first one applies only if you recorded them) FX, at least (I don't know about other stations that syndicate Buffy) cuts lines out of the eps...which is very annoying, because they're usually amall lines but funny and show the depth and style of the show.
Secondly, I personally love commentaries that are on dvds. They give you insight into there characters and writers' thought process, and help you understand the show on a different level. This was my major motivation for getting the dvds...I love stuff like that. (I wish there was more of this on both season 1 and 2....but what's there is neat.)