Wow. You are getting some incredibly good prices there.
I am seriously biased, so dismiss what I am about to write.
When I was shopping I visited the dealer specifically looking for R300 because my son had just picked up a pair of Q300 from Accessories4Less.com for reasonable cost. They were better than the iQ10's he had given me the year before, and about as good as my old Hafler 200's. I wanted better because...I'm the dad. My son was with me when we went to this dealer I'd never heard of to audition the R300. They were out of them, saying they'd recently sold the last pair in stock. However, the salesman said they could setup the R500 to sound similar by running through an AVR with Audyssey, setting the low end to a little higher than what the R500 can really do and just disconnect the sub and put the lower port plug in.
We were just so impressed with the speakers that I wanted to order a pair of R300 on the spot. My son said they were definitely better than his Q300. But the salesman wasn't done. He connected the R500 to an NAD C375BEE integrated, no bass management and port plug out. My son and I sat alone in that room for another 45 minutes playing anything we could think of that we knew from his and my "internal playlist." We were absolutely blown away. The R500 was just in a totally different league than the Q300, and presumably the R300 too but there's no way I could at that time cough up the $2600 for those. Now the salesman offers me the demo model R500 with full factory warranty for the same price as new R300. For the sake of a couple of tiny nicks that you have get close to find, I bit.
These R500 continue to fascinate me. There is more bass in these than a small floorstander has any right to have. I often demo my speakers with the subwoofer off and then turn it on and that surprises many people. Probably 90% of the music I listen to never engages the sub at any noticeable level, so more often than not I put the receiver in analog bypass mode and send full range signal to the R500's. I do have optimal placement from the rear walls, about 18" to the back of the speakers, and that helps. My son comes over often and continues to comment about the profound difference between his and my speakers. (I recently got him a pair of Q500 so it's subsided a little.) And I know there's more in these as I upgrade my source from an NAD T758 to a quality integrated in the future.
The price you have there for a pair of R500 is only $100 more than I paid for 2 year old demo models. You would NEVER regret getting those over the R300. It's the LS50 or those now and a tough choice, but think of it this way: If you got the LS50 you absolutely need a subwoofer for far more of the music and 100% of the movies, but you have $800 in your pocket to upgrade or add one. With the R500 it's not so important to have a sub for everything, and they just sound so full compared to standmount speakers. Buy the R500.
OK, so now you have my long-winded thoughts on the matter. Dismiss my overly biased opinion.