The ATP I would be doing in a multi because I need the multi time anyway. As for the MEI, I am thinking less of it now because I have no way to use it locally, and am hoping to begin the application process in the next couple of months anyway.
Personal opinion:
Here's your flow chart as I see it: Multi-time > ATP written > MEI > ATP.
My current employer (pt. 135) hired me 60% on a personal qualification (we know you, and trust that you could represent us appropriately, 100% of the time) and 40% on fact that I had specific ratings and qualifications (Multi-time!!!!, flight instructor including MEI,) and they blatantly told me so after hiring me. Mostly, the ratings mattered to them because it provided a higher insurance that I wouldn't have any training issues once I was sent to school; subsequently the ATP was not on that list.
ATP's, in my opinion, don't make you more marketable in most instances. Only that you qualify for the ATP, and one of those qualifications is that you have the written completed. After you pass certian career milestones, it is implied that you have your ATP; and the ATP is most often given to you by the employer (think if it as a gift!). Why pay for something you know you will be given down the road?
Obviously it should be viewed from the standpoint of "what is required?" over "what would make me look good?". What won't make you look good / hireable, at all, is not having what is required, e.g. multi time.
You really don't NEED either the ATP or the MEI. You really DO need the multi-time. The ATP is not, and should not be a catalyst for getting hired if you don't ALREADY hold a type rating for that aircraft to be flown. When you get the type, you get the ATP, and the only thing that changes is the check mark that is included on your paper work. If you get into a plane that doesn't require a type rating, you still have to take a check ride, and again the ATP can/will be included in that ride.
ATP's don't imply you can fly twins, but multi-time certainly does; and the more the better.
Consider this! If you did get your MEI, you are still limited to the avenues in which you could gain multi-time. If say, you wanted to rent a twin from somewhere or someone, so that you could give multi-instruction, chances are that the place providing the airplane (e.g. a flight school that also rents, or a guy you know), is going to require that you meet their insurance minimums. I can tell you from encountering this road block myself, that number can be a huge limiting factor as it is in the hundreds of hours (especially if you intend to give instruction in the plane), and is much higher than what any regional will be asking from a qualifying FO.
If you are hell bent on getting another rating, I'd go MEI. The ATP will land in your lap with a bow tied to it, when the time is right. The only thing that matters more than a personal recommendation are the numbers at the top of your resume, and if you don't have those numbers you don't pass "GO". After that, all that matters is not being a complete idiot / safety risk.