Should I buy Nexo or Celsius Token with Bitcoin?

You may want to buy Nexo token if you want to increase the loyalty level and in return Nexo will give you more APY.

I am planning to buy stablecoins and swap it with Nexo token to increase my current loyalty level with Nexo, just to get a better APY.

You want to swap the King of Crypto for a centralised altcoin??

No!

Use stablecoins (eg. USDC/DAI/BUSD) to buy Nexo or Celsius, Bitcoin are meant to be held and stacked over the long run