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.