RSK is a sidechain of Bitcoin. As more BTC are given to the sidechain more rBTC are minted, while if you burn rBTC you can get away with BTC again. rBTC prodives turing complete. It's like a programable BTC, the price is attached to BTC.

STACKS, is proof of transference. They use their own token called STX that also provides programmability. To produce each block in the blockchain of Stacks you need BTC. The difference is that Stacks has its own price (variable).