Personally, I'd wait to buy a piece until you get a drop, then you can buy two together. Why buy a gun, and then when you get a drop, it's for the gun?
Failing that, I'd go with the assumption that Cure Ground is the hardest to beat, so the least likely to run though as many times. Buy that piece, so when you get drops from Infected or KA, you aren't getting a dupe.
That's my general plan, anyway. Got a Mk XII shield, so grinding Cure and KA to try and get more of that set. For the ground missions, pretty much just going to buy the set. I've almost got 80 chips to cash in, but am going to run Infected and KA a few times, and see if I can trick a drop out of the system, and then guy whichever is missing. Pretty confident buying the Cure ground piece, though. Not a matter of which piece is best, but which one I'm definitely not getting dropped anytime soon!