I don't think the question is "Soundbar vs Center Speaker" but rather "Soundbar vs 5.1 speakers".

I haven't tried a soundbar myself, but from what I have heard it can give you a pretty decent sound. But it'll never give you the same surround feeling as you get with real rear speakers.

If you have a problem putting up "real" speakers, a soundbar may be a good compromise. But it will never give you a sound comparable to 5 separate speakers (and a sub).