Both are dope. I have a link to the new Roc Marciano, but his LP is VERY disappointing IMO, but it's simply because I don't like the new direction he's goin in. His voice is very 90's east coast grimy but his beats are on some weird shit.
Blacastan is most well known for his concept single "Life Of A Tape" basically about a casette. Anyway, he's a close friend of Apathy's and gets shine off that as well. His new album should be very good.
Both are dope MC's, but album wise peep before you buy. Blac has a very distinct style, and Roc definitely isn't for everybody. My son (who's 2) literally sang a long to the hooks of one of his songs "Do it, do it, do it, do it".