Sorry for late response - I've been out in the wilderness playing the tuba.
I've only listened to Polk 6501 speakers for 10 min in the shop - comparing them to the Infinity 6520. In the shop the Polks sounded best to me. May be different in the car tough.
First impression was a darker, more punchy and slightly more easy listening sound in the Polks. The Infinitiy speakers created a more "clinical" or "technical"

sound with more strident tweeter. The Polks also cost 25% more.