I upgraded from a 172XS which I owned for 2.5 years, to N272 in June 2015, see my post under "N272 Owners... How's Yours Sounding?" dated June 25, 2015.
During a back-to-back comparison during a weekend at home, the 272 appeared to have a slightly warmer, more natural sound and a very natural beautiful bass/low. The highs are slightly less distorted and more clearly defined than172XS, which became evident in the cymbals on some records. After a couple of weeks the strengths of the 272 became more clear and I really enjoy the very natural and neutral SQ since then. Overall it is slightly better than the 172XS in a line-up with NAP250 and B&W802 Diamond.
But to be honest: if you wouldn't know the difference, you would be a very happy 172XS owner. The only advice I can give you: give your dealer a smile and ask for a demo N272 for a weekend. Get a friend who will change-out the cables between 172-272, without telling what you are listening to, for a true blind test. If you struggle to tell wether A or B is better, don't bother and keep your 172XS! The money is probably better spent on other componenents e.g. your speakers. In every test I have done, the speakers were the most determining component in overall SQ.