I tried it thoroughly, when the Library came out. I do searches in foobar, writes in Mp3tag. So this is the fastest way for me, i tested it.
I wrote above:
About once a month.
For me, the Library would be worth, if there were those drop-down lists, based on it. Then it would be faster, than always to jump to the foobar. Which, by the way, is not so common way in my practice. What i need to copy, i open (most of them) in Mp3tag. I use a lot of export + import, for example like i described here: Data in REAL database, instead of tags?
Often, I modify the exported aaa.txt file in EditPad (or Pro) first (also a great program), before importing it into the new files.