A rack unit would be very cool, particularly considering my fabled "Super Rig" concept - which was a Helix Rack + Axe FX III + Mark Five amp, with either one unit in the FX loop and one unit in the front, or both digitally connected together via SPDIF and then the whole thing in 4-cable-method; with the Axe3 slaved to the Helix via midi program changes or control changes.
Massively overkill, but would be amazerocks!!
On balance, if I had the choice, I'd go floor unit as a standalone thing. But racks do appeal as well.