Quick background here -- Worked at IBM for almost 20 years, and with other professional services groups of various sizes for another 10.
The CbD (Consulting by degrees) program is a good "whirlwind" program to get exposed to a lot of different projects in a short amount of time. We used CbD'ers on several of my projects and they were top notch folks and really helped the project out. Tasks may be a little more administrative, but you will get exposure to people, process, and technologies. So, I would give a positive review to that.
In general and in my experience, consulting (anywhere) is a little like being a technical gypsy -- you move from project to project, and your successes come from effective and efficient projects, and being well utilized (being billable to clients as opposed to working on internal projects or being on the bench). As you move out of the CbD program, you will probably be asked to join a practice or service area which meets your qualifications and business needs. As you do, hit the ground running, and get billable.
As per company "a" vs "b", there's a lot of similarities in this field, and in my experience, the job satisfaction and quality of the experience is a little bit of a dice roll, but mostly what you make of it. The good news is that most projects that I've been on have been in the 3mo-24mo range and you get to change your venue (whether you like to or not) and move on to different clients, different projects, and different locations. Very rarely have I seen long-timers be in the same client location working on the same projects.
Personally, as IBM goes, it's an awesome company, it's not like it was "years ago" with cradle-to-grave career mentality and a pension. It's competitive, it's heartless, and it's methodical. If you're lucky you'll meet some awesomely intelligent people, work on some challenging issues, and learn a lot.
However, you are personally responsible for managing your own career path. Pick your head up after 2-5 years and personally evaluate if the business relationship between you and your employer still benefits you the way you want, and if not, move along.