Very odd you had to wipe twice...bizarre, really, a wipe should be a wipe. Oh well, you resolved it.
You can move between CM7 and MIUI at will, as MIUi is CM7-based. No SBF required.
However, you should wipe data between the two the first time (e.g., if you go to MIUI, wipe data the first time you install it).
I have a Nandroid backup of both CM7 and MIUI and can switch between them at will (and am doing so no) when I want to.
MIUI currently has connectivity issues that mean that you may lose all connectivity (data/voice/text) intermittently, and have to reboot to reconnect, so I'm going back to CM7 until those issues are resolved. I would hold on trying MIUI until they fix that stuff, but you can try MIUI if you want of course - not everyone using it is getting the connectivity problem.