Do you have any input on how these compare to the power rs?
Bang for the buck, I think the S21 + rebate wins. But it depends on the riding you do, or more specifically, are you actually going to push the tire anywhere close to it's limit on traction.
If you're a newer track rider or just a canyon carver, you can ride with S21/Q3+ just fine and will get longer life out of them.
Power RS should be higher traction than Q3+, but with worse wear.
In my opinion:
Tier 1: Racing slicks
Tier 2: Single compound/track day slicks
Tier 3: Power RS>RS10 (but close)
Tier 4: S21>Q3+ (but close)
I've ridden both S21 and Power RS, both are fine tires. Power RS rear lasted about 500-600 track miles on a 600 with relatively gentle throttle coming out of turns, so pretty good. I like the front better than the rear. The rear didn't seem too difficult to break loose in the last 200 miles/multiple heat cycles. I've also seen people using Q3/Q3+, and have ridden Q3's, but never pushed them.
If you're not pushing the tires, just buy whatever is cheapest. It's not going to matter, so save the $$$. All of them are fantastic tires.