I think that if you are interested in working specifically with problems related with DM and obesity, and not so interested in ID, GI, Cards, Renal - then it makes a lot of sense to do two more years and increase your fund of knowledge in the area that you have the most passion for. Yes, you get plenty of DM-related issues, hyperlipidemia, and obesity as an internist, but you also get plenty of exposure in other areas. To me, wanting to focus on DM and obesity is no different than someone wanting to focus on GI --except for the paycheck. And I have time and time again from most of the docs that teach us here that while your paycheck is important, its value will not be so great if you are miserable in what you are doing. Gotta do what moves you.