+1 for preferring TXXX, since I use metamp3 still for my ID3v2.3-tagged MP3s (TXXX only, no RVAD/RVA2). Due to the conversion to ID3v2.4 I cannot (yet) use QL for RG'ing.
More reasons:
I broadcast using IDJC and IDJC also prefers TXXX over all others.
In case both are there, prefer txxx, as that's what foobar writes.