I haven't spent a ton of time with the on-walls yet, but initial observations are that IW don't sound as full, lacking in midbass, imaging was hampered (probably due to proximity to wall-mounted TV). IW are not ported.

OW and bookshelf are very similar, with the OW rolling off sooner on the low end (and also being easier to tame). ARC Genesis wants to cross both at 80hz. OW might actually sound better than a sub-optimally placed bookshelf.

Axiom M5HP VP160 M3 ADA1000
Anthem MRX 720
SVS SB-3000 (dual)