For the record, the MEI was a crucial element to me building multi time. I finished the Commercial multi in 7 hours. To buy 93 hours of multi time to get that magic 100 would be, well, prohibitively expensive. (93 x $200 = $18,600). I am fortunate enough to work at a flight school that payed for my MEI, but even if they hadn't, it surely would have been cheaper than $18,600. (Incidentally, $18,600 is about 3/4 of what I payed for my private through CFII.) It seems to be that it would be wasting money to not get the MEI, not t'other way around.
In any case, I think you'd be better off flying at an FBO, provided that it was a good situation, i.e. an experienced and skilled instructor, an airplane that is in good shape and reasonably priced, FBO management competent, friendly, professional, etc. This is nothing against ATP per se. I haven't really dealt with them at all, so they may be great for all I know. I just happen to feel that you'll save a lot of money in the long run if you allow proper time to absorb the information in the first place, and there really is no better place than an FBO to do this. Take your time and actually try and learn something.
-Goose