See the CentOS wiki pages on package management. Both are good, neither should conflict with base, but they will conflict with each other.
You can poke around that package management section and see how to use priorities. I would use it that way, probably giving epel priority over repoforge.
Unfortunately, they all have something the other will lack. If I were going to use just one, especially on server level, I'd probably choose epel, as the Dag repo (repoforge, aka rpmforge) often has more desktop oriented stuff. (This generalization is not always true.)
You can always have both and leave one disabled (by changinged enabled=1 to enabled=0 in the repo file. Then, if you do need something from it, you can enable it on the command line.
You can use both of them, but if you're inexperienced, it can be a bad idea as one will install something, then, when you next try to upgrade, you see the other wants to upgrade library X which is used by program Y but there are conflicts.