I started with M60/VP100 and was thinking about upgrading to better handle the REALLY FREAKIN' LOUD playback levels some of my friends seemed to like. Then the VP180 came out, I picked up one of them, and from that point upgrading has never crossed my mind.
The significant impact of the VP180 got me started doing some experimentation - walking up close to each speaker to see how much content came from the centre channel vs mains. My conclusion was that for typical modern HT content there's a lot to be said for having a center channel that's "a bit bigger" than the mains. Not all content needs it (some content doesn't use the center much at all) but IMO you should *definitely* look at upgrading the center channel.
I haven't heard a VP160 but did spend a lot of time bugging the nice Axiom folks to make something just like it, at least until the VP180 came out. AFAICS the "mids in the middle" driver arrangement on the VP180 takes care of practical comb filtering concerns, which I believe was the primary argument for VP160 layout being theoretically better than VP180, so I would still lean towards the 180 if you have room. It is pretty big