I have a BK-7m, and it's an amazing module. The display could be bigger, but it occupies a unique space in the market. In that sense it's amazing piece of gear that was well worth its street price when it went out of production.

I also briefly owned a BK-5. It was nice, but IMO not as nice as the E-50 that preceded it, so I kept my E-50 and have no regrets. The biggest plusses of the BK-5 vs. the BK-7m are a larger screen, and an on-board Style Composer.

The BK-7m does have Roland's Makeup Tools for revoicing, etc. But there isn't a built-in way to create styles from scratch. A primitive "Style Converter" software was bundled with the -7m. As I recall, it didn't have a way to insert the "Alteration Mode" messages which establish the upper and lower note limits (aka "wrap range") for each style part. These alteration mode events also determine whether a Roland style part transposes "in parallel" or uses a "close" voicing, which makes a lot of difference in the resulting sound! Trivial to most here, but if you're into style editing, you'll prefer the BK-5.

You don't mention the E-A7, which is a couple of years newer and probably the last true Roland arranger. Compared to the BK-5, it has two non-touch displays and the possibility of multipads (you have to develop your own sequences for the pads.) However, the BK-5 is still available new from major online retailers at a slightly lower cost.

Roland is my preferred brand. The styles are highly playable and their chord fingering is easier than any other brand, without penalizing highly skilled players. It's a shame that they have pretty well abandoned the arranger market. IMO the BK-7m, BK-5, and E-A7 are all good instruments and worth owning.