My opinions differ a bit.
I've owned the Ketron X4 since 2001 as well as the BK-7m since 2016, the sounds are a bit better on the X4, but it's not as easy to configure and use as the BK, lots of initial messing around with the X4's channels and what not, there may be things that will drive you crazy that I had to really research... like the right hand volume following you on the X4, however it over exaggerates the changes, and is not as linear sounding. Once a few settings on the 8X are disabled it's a lot better.
That said, though the BK can take under 1 minute to set up the first time (actually, well under 30 seconds and near zero knowledge, just follow the prompts for the quick setup!), the X4 can take about 5-10 minutes the first time if you have the manual for both the 8X and X4 and understand what you are doing, after that, no messing around with either... turn on and connect to V-accordion and enjoy.
In terms of size, the BK can easily be held in 3 fingers and is about 1/3rd the size of the X4 which is much heavier, requiring 2 hands to move it around.
Because the BK is a Roland and the 4X and 8X V-accordions are also made by Roland, they were basically designed for each other by design, so I found the BK way easier to use with the 8X than the X4. Things like over 900 integrated songs, ability to create your own performance lists, ability to control the arranger from my music display software become very important once you start using it (and many more things) all add to the experience.
All that said, once you know both, there will be things that you like better with one over the other and things you will hate with one over the other. Neither is perfect, neither is in production anymore and pricing really is going to depend a lot more on luck than consistency (I paid $2200CA for my X4 brand new out of the box the first month they came out and around $675CA used for the BK).