Since my main use will be on the C172, how much of any advantage will
the 20XL be over the QFRs? What would be the main difference btw the
20XLs, and anyway to tell if they are of the newer type?


The difference will be clearly noticable. The older ones are not sold
anymore. I think they are well worth the extra money, but as you can
see even in this thread, others differ - and only you can find out for
yourself. That's what money-back guarantees are made for. Or a local
pilot shop.

--
Thomas Borchert (EDDH)