I have owned the Infinity Primus and AJ Pioneer speakers, as well as, lots of Polk Monitor and RTi series speakers. In fact, I am listening to my new RTi6's along with the CSi5 center as we speak. There is no doubt the Polk RTi series is the much better choice.
Polk speakers have a certain sound signature that YOU will either like or YOU won't. At least, that is what I have found. The Infinity Primus speakers are good for the money. But, I highly recommend YOU go give the Sony Core speakers a listen at YOUR local Best Buy. They are much better than the AJ, Polk Monitor, and/or the Infinity Primus. When on sale, they will be hard to beat imho.
Good deals on the Polk RTi series of speakers can be had on the Polk Ebay site. Newegg also sales a lot of Polk speakers via Ebay. Have bought from both and I highly recommend them. Good luck with YOUR choice.
Cheers,
Phil