I've never heard either speaker but the Alexx V is a much newer design and every report says the newer Wilson speakers done since Daryl came in charge are superior speakers to David's designs(I once emailed Ken Kessler and he agreed) and they have had the benefit of 10 more years of development. The one advantage of the XLF would be better ability to fill large spaces and a few bass Hertz. Otherwise the Alexx v is a significant sonic improvement.
One caveat. Choosing between good speakers also involves taste since none of them are perfect and their pluses and minuses usually differ. So as usual if you can listen to both as much as possible before choosing.