I think it’s the same, or at least I cannot tell. I have both v6 and v7 dudes running, nothing is different and both work.
It is actually mixing Dude versions that’s actually more of reason not change… if working. The Dude 32-bit client app does an update when you connect to a Dude with different version, which generally works even under wine. But OSes don’t like swapping EXE tricks Dude pulls, and ain’t specific to moving to V7… v6.48 to v6.49 be same issues with client swapping EXE, as it really wants versions to match.
Anyway I wouldn’t let The Dude guide the upgrade choice, I have not seen any difference between V6 and V7. But if you needed some new RouterOS feature, that be a better reason.