1- Why the price of the staking is variable and not constant?
that depends on the project and various factors. The price of token will be also making staking become variable.
2- Which coin is good for the staking?
I personally prefer to do farming rather than staking. This give you option to farm the token with good APY with your stable coin although that you can stake your stable coin as well but the APY was so small compared when you're farming the tokens use your coins.
Staking reward doesn't seem to be accurate and you can check the latest APY that being offered through this link
https://explorer.smartcash.org/home