Hello !
Mp3Tag suggests the meta tag "ALBUMARTIST", but Foobar2000 only recognizes "ALBUM ARTIST".
Who is right ?
pihug12 
Well, for MP3 files, AlbumArtist is written as a TXXX frame, so there is really no difference with Album Artist in that regard. I'm assuming then, that the difference is what name the player is using to read that tag. So, WMP uses AlbumArtist, while foobar2000 uses Album Artist. If you use foobar2000 then stick with Album Artist. To answer your question, both are correct depending on which player that you use.
I have the same problem: mp3tag does not support ‘ALBUM ARTIST’ Tag (with a space)
What do you mean by "Mp3tag does not support ALBUM ARTIST"?
Where do you like to see the content of it?
Or where you like to add content into ALBUM ARTIST?
BTW: The official ID3 field name ist ALBUMARTIST.
Perhaps this topic will help too.: Album Artist vs ALBUM ARTIST
Please check the extended tags dialogue Alt-T and you will see the field ALBUM ARTIST - which would be a user-defined field.
The important bit is that the field is stored as TPE2 in the metadata. If it is stored as TXXX:ALBUM ARTIST, then it is a user-defined field