If you don't need to buy mats for faction rep then I'd suggest buying weapon engrams from the cryptarch. That's what I do, you never know what'll drop, it could end up being something God rolled. If not, dismantle and get back three legendary marks and start the process over again. The cryptarchs in the tower, reef and iron temple all tend to have different inventories, so if one doesn't have an engram you want make sure you go check out the other ones.