performance differences between 3B and 3B+ devices will be absolutely negligible when it comes to basic scripting (midi/etc) and softcut performance.
the slightly faster cpu on the 3B+ may allow you to design supercollider engines which don’t run well on a 3B, however. leveraging this tiny improvement is deeply discouraged given the goal of this ecosystem is broad shared support. this is the reason we’re not officially supporting the pi 4: it splinters the community.
so, any slowdowns you’re experiencing with midigrid is a bug, and should be reported.
the takeaway is: get a 3B or 3B+, it doesn’t matter: our goal is to support both and have them act basically identically.
(takeaway: if you feel you need to squeeze that extra bit of cpu and run a custom supercollider engine right up to the edge of capability, please keep it private, or share a more broadly-stable version).