What should I expect in terms of the differences? If I wanted to go with a hollow body (on account of I have a solid body with the DiMarzios already) how would that play a role?
Two very different animals honestly. P90s are single coils essentially, and the dimarzios are obviously some high output humbuckers.
That said, if you already own a HH axis, and craving something "different", then by all means try out those p90s. Also, remember you can always add p90s to a humbucker route, but not the other way around. Seymour duncan p-rails may be something to look into.
I enjoy both versions...for me ,the ideal guitar would have the MM90 in the neck,the HB in the bridge...the neck MM90 is creamy clean,the bridge MM90 has lots of snap to it,but not the fat drooling oomph of the HB....I find the neck HB ok,but not as articulate as the MM90...But,they*re both great..I play country blues/rock..so the MM90 wins out.