I believe the BK-5 doesn't have the 'Wizard' setup tools that come in the BK-7m, so setting up reception channels, controllers and the like might be a bit initially more difficult on the BK-5, but there are several performance tools with buttons on the front panel of the BK-5 that are missing, buried in menus, and possibly not MIDI trigger-able in the BK-7m.

I guess it boils down to just how familiar you are with getting different things to talk MIDI with each other. A good controller ought to get you possibly MORE control than you can get out of most modern arrangers (none of them offer customization of what the sliders, if any, do, whereas with a good controller, you can CHOOSE what you want, where you want it, etc.), but at the cost of some serious head-scratching as no two companies ever seem to document their MIDI capabilities in the same layout and language.