I do agree that most KEF speakers offer higher sound quality than most Polk, though also agree that once you get into Polk's higher end products they are very competitive. The higher cost of KEF is why they retain higher value. Even Polk's LSi series doesn't seem to command much respect on the used market, so as long as they're bought to keep it's no problem.
While I endorse KEF, listening to both would be advised before making a decision if at all possible. If it's the LSi703 stand mount in consideration (street price about $1500 a pair), then the only KEF at that same point would be the much touted LS50. It won't have the bass response of the Polk, but blows the Polk away in every other area. A subwoofer would be required. Jumping up $300 to the KEF R300, the Polk LSi703 has met it's match in every way except price. For a lot of music, a sub might not be needed.