I was in the same shoes as you last week. I've been using MM for over ten years but MB, so far, has been awesome (a lot of ways to customize). The pic below contains Mediamonkey's custom tag fields that you would probably like to sync. MB transferred ratings and all of the custom tags (provided you follow what is seen below). The only thing that MB did not sync were playcounts. Luckily, I always scrobbled my plays through Last.fm and MB's Last.fm playcount sync is far better than MM's addon (the latter gave me wildly high playcounts on many tracks; luckily I backed up mm.db before initiating it).
Remember for mp3's, MM's custom fields are COMM/Songs-DB_Custom# (this was a typo in MM's tag chart:
http://www.mediamonkey.com/sw/webhelp/frame/index.html?abouttrackproperties.htm). I would first enter in the data for custom fields as shown in my pic below and then import a directory or two and see if the tags and ratings imported correctly. After that, import the rest.
My MM.db is 1.7GB and MB's db is only 300MB!
EDIT: I forgot to add that your initial testing should include albums containing tags you want to import. Highlight a song, shift+enter (or right click > edit) and hit tag inspector (red arrow: see below) to see how the tags are formatted (pay attention to the tag code). Afterwards, you can create custom tags and those values will be imported in short order. When you write into those tags in MB, they will show up in Mediamonkey too!