I have both colors and they seem to be equally hardy. If buying from a LFS, you may be better off with the red ones. The collection practices on the blue's are known for being very harsh and they usually die (for no other apparent reason) within 6 months of their purchase from a LFS. If you see someone selling one on the forums that has been in their tank for 6mos. or more, make sure to jump on it asap, even if they are charging LFS prices for it.
I have 7-8 red linckia's (most are just legs) and 2 large blue linckia's in a 29 gal at the moment, waiting for a tank upgrade in progress. If you decide to keep it in a 20L, then you may have to target feed it w/ some of Rod's food, b/c you probably won't be able to feed the tank heavily enough for it to get enough food off of the sand without jeopardizing the water quality.