Indeed, holding DEC has the best return in terms of Airdrop Point/USD. In the other hand, I would not hold it for this sole reason. DEC is meant to be pegged to 1 DEC = 0.001 USD, and now it's worth almost 10x more (around 0.01 USD). It's value is currently high (and will probably stay high for the next several months, maybe even more) due the high demand for Airdrop Points and Cards. But, sooner or later, the price will come back to near the pegged value.
My personal strategy is using my DEC to buy cards with high Collection Power / USD rate, mostly gold-foiled cards that aren't really used in matches. Yes, I'm losing part of the Airdrop Points, but I'm also holding something with a high chance of increasing its price over time.