I suppose it also depends on your goals for the MBA and what concentration you are interested in pursuing.
A part-time program puts career switchers at a disadvantage in terms of access to on-campus recruiters and the time for an internship. Further, your ability to network and interact with other students is limited by the demands of your work.
On the other hand, as everyone points out, the PT program allows one to reduce the oppty cost of attending school by continuing to work.
Further, it is unclear what you mean by a "B" grade school - those set on Wharton or Stanford might consider some of the world's best business schools "B" grade compared to their favorites. Further, some of the TransElite schools such as Carnegie are extremely strong in certain fields even if they are generally perceived as less elite than the "Top 10." Size, however, seems to have little to do with the quality of the school. Of the sixteen or so "Top 10" schools there are several large schools (such Harvard, Wharton) and several relatively small schools (such as Tuck, Yale).