I am not a financial expert, but I only keep DEC and every day I convert the SPS I get from the Splinterlands airdrop back to DEC.

The benefit from DEC is that you can convert or withdraw them at any time you want, while SPS has to be unstaked first before you can convert or withdraw them.

I am of the opinion that you make more money by reinvesting SPS back to DEC then staking SPS, but I must admit that DEC has decreased quite a bit in value, while SPS remained stable.