Etaoin_PigI have both, and the K553 is better in SQ in pretty much every aspect, to me.
The K550, by comparison, sounds like it has a slight veil over the sound, muting some of the detail slightly. The K553 seems to have better clarity and detail.
Mind you, they're VERY close to each other. The K553 just seems slightly better in almost every way to my ears. My one nitpick is that the pads feel slightly larger on the inner diameter on the K553, which causes them to touch my earlobes, where they do not on the K550. In this way, I find the K553 *slightly* less comfortable than the K550, but it hasn't hindered me from long listening sessions as I feared it might.
At the same price level, I'd choose the K553 over the K550. At the Massdrop price, it's a no-brainer to go with the K553. My K550's are going on Ebay.