What is the difference between version 6 and 7 in MikroTik?
That's like asking: what's the difference between car with electric engine or combustion engine ?
They are both cars. They both can get you from A to B.
Basically:
ROS6 is not actively developed anymore except for security fixes
ROS7 is another kernel, new features are added, different way of handling things behind the scenes, ...
All new released devices (and a lot of older ones as well) come out of factory with ROS7 nowadays.
However, there are older devices which are not really suited to run ROS7. Those are best kept on ROS6.
If ROS6 works for you on your current devices without problems, you can keep using it.
Unless you need new features which are only in ROS7, then you need to see what device you have.
Rule of thumb: wireless AC devices with 16Mb storage are dangerous territory, unless you keep using legacy wireless driver.
Devices without wireless will typically be able to run ROS7 (but they may loose a bit of performance because of the way things are setup in ROS7)
Mikrotik made only one hard rule:
Note: We do not recommend running v7 on hardware that does not have at least 64 MB of RAM.
v6 = long-term or really stable, developed for WISPs and for those who work.
v7= no-long-term, and not really stable, he doesn't care about WISPs, he doesn't care about what has already been installed, developed for geeks, and for beta-testers, whether voluntary or not...