The quality of the center speaker by itself is IRRELEVANT if it does not tonally match the L/R mains. Before you ask which center speaker is better, you should look at which main speaker you're using and ask which center speaker model matches the mains better. A center speaker with noticeably different tonal characteristics will stand out and detract from the seamless panning effect you want from the front three speakers, even if it subjectively sounds better.
I agree with Woochifer. Of course it's not irrelevant if you own speakers that don't have a matching center speaker or the matching center for your brand is truly horrid.
80%+ of the dialog in the vast majority of Movies come Directly from the center channel and while panning it is desirable to have a match...life isn't perfect even if you have a matching center...they don't really MATCH...they may be the best match but certainly not a perfet match...that requires the center speaker to be EXACTLY the same as the front left and right...so now you're talking about the Best match - which is then subjective to the listener. Generally speaking however the Paraidgm will work best for Paradigm and B&W for B&W. But if you're using JBL say and there was no matching center then by all means find the center channel that produces the best dialogue and music.
Your panning will be a bit off but I'd take a bit of loss for 20% of the panning across the front if 80% of the dialogue in the center is done correctly.
Frankly, If it were me I would not buy a non matched center channel and run it in phantom mode. 2 channel stereo places singers dead center...and my DVD collection runs around 200 and I dialogue pans well left to right and in the center - despite the mix downs. Phantom modes if they're good and if you have good speakers should pan left to right and not require a center channel...at least not to make out dialogue.
As for which one is better that is going to depend on what you're connecting it to and your taste. I've heard the B&W's in a B&W system and it was very solid. I've also been more impressed by some of the Paradigm Center channels than I've been with their front speakers...strange as that may seem.