I guess the way I think about it is that if your company didn't need you anymore, they would not show loyalty to you; they'd let you go. In the work world you have to look out for yourself first because no one else is going to. If there is a better opportunity elsewhere, then I'd look into it. People generally are reasonable and undertstand if a better job comes up you'd want to take it. (Well, one boss of mine didn't handle me leaving well, but he had a lot of issues.)

If the other job does work out, then I'd say try to leave your current employer on the best of terms. Burning bridges is never good. Give adequate notice. I usually also try to leave things I was responsible for well documented for whoever comes next as well. Good luck!