What about keeping your SPS and using anything you're winning in the game to add to your playing deck? We should have a few months at least to be buying the cards with favorable buyer prices and this way you can focus on your strengths getting leveled up to the league you want or filling in any weakness that may be holding your game back rather than hoping for random chance to provide the cards you're needing. STDD also had a good suggestion if you're uncertain about what to do, splitting up your daily airdrops can be a helpful strategy.

I'm currently staking somewhere between 60-70% of my SPS airdrop each day and putting the rest aside to use when Riftwatchers is out so I don't have to add any more money to the game at that point.