I was taught that mortise chisels, except for really deep mortises more than 2" or so, are unnecessary. The thicker frame resists racking and bending while levering out chips, which is really not an issue with 90% of all mortises. I have a set and honestly there is no difference in the cut or the time spent chopping them out. Save your money unless you are timber framing.
Paul Sellers' Opinions on the Subject: https://www.youtube.com/watch?v=q_NXq7_TILA