An RX 460 should be a decent amount faster than an RX 550. If they are priced similar, then the RX 460 would be the better option between the two.
Another option that you might be able to find around the same price range would be the Nvidia GTX 1050 2GB. It has less memory, but in most cases tends to provide similar or better performance than the Radeon RX 460. And if you are willing to stretch your budget a bit further, the GTX 1050 Ti 4GB will be faster still. After that point, there's currently a fairly large price gap to the next higher performing cards.