Just came from x7 to x9.
The main mounting pivot on the x7 normally has loads of slop from factory(had a fondle of a few), the pivot design also gets a lot of slop easily, this results in the mech sitting at a different alignment to the cassette dependent on how far up and down the cassette it is.
The x9 has a slightly different mounting pivot configuration, has no slop as of yet, the different configuration also look less likely to develop slop.
The x9 works better from experience, but can’t comment whether this will hold true a year down the line.