In my field, we use a lot of battery backup systems. I like the 24vdc powered version for that (combined with a PULS buffer module and a big SLA battery). We always have at least a few 24vdc sensors so it makes sense to use the 24vdc powered version of the Micrologix.
I don't think there are any significant disadvantages to using 120vac powered Micrologix unless you don't have 120v available.
I have one installation where the incoming power is sketchy and the 120vac (ML1400) model lost its marbles a couple of times...probably due to a brownout. The plant operator added a household grade UPS to "cure" it.
I am not sure if they're more sensitive to transients on the AC line, but that is anecdotal at best.