The 5 and the 3 don't have quite as many sounds and kits (you can still download their manuals if you want) but I think the primary advantage of the BK7-m if you are using it as a remote module is the far better MIDI implementation. None of the keyboards have the mode where you feed it one MIDI channel and it maps the Performance's splits and layers across that one channel.

This complicates your controller setup, forcing you to create the splits and layers on the controller for each Performance. If you want to avoid this hassle, or tend to use a couple of different controllers from time to time or want to future-proof yourself, the BK7-m's single channel remap system may be your best bet.

I've messed with a 3 and a 5, had them next to my BK7-m, and overall I felt that the sound seemed a bit thinner with the keyboard versions which is pretty much the trend, sample loops get a little shorter, D/A converters get a little cheaper in low end products.

Much harder to hear the difference, identical sound for identical sound between the BK7-m and the BK-9, though.