Some say using a 220 back with 120 film creates a film flatness issue (the 220 film does not have backing paper through the entire roll, so the overall thickness is different). Others say it's a theoretical issue that does not have practical implications.
BTW, a similar argument is made over backs with matching inserts vs. those with non-matching inserts.
If I was looking to save some money, I'd buy the older C12 back before the A24. But that's just my opinion.