I am a staunch supporter of 141 schools or any other kind of structured, organized program.  In this situation, unless you're already enrolled in a 141 program and/or do not have your CFI, I would just go the Part 61 route.  The MEI is the easiest CFI add-on.  The oral will likely focus heavily on systems and Vmc, though you might be hit with a few FOI questions.  You can already fly out of the right seat, so it is essentially a Commercial-Multi from that seat.  Of course, some examiners might ask you to demonstrate some instrument work.  But, otherwise, there really is no reason to spend the extra money.
I just would add that there are people and programs in which the initial CFI and CFI-I is earned in the multi.  I think that's a great plan, especially if you are fresh off earning your Commercial-Instrument in the multi.  Do that, and all you need is a simple single to add your Commercial Single and CFI-A to your certificates.
Hope that helps some more.  Good luck with your MEI.