I would go with the ME rating first, for the following reasons:
1. Turbine time is turbine time. 2 hours in a Merlin is better than nothing, just be sure you know your systems.
2. Save yourself a checkride. You can do your initial instructor as a CFII in a single, and then add on the MEI rating. A bit of a switcharoo on the old CFI, then CFII, then MEI routine.
3. (The most important one) MEIIs are more marketable than CFIs. Most guys that start CFIing with no II or MEI never wind up getting them because, hey, it's hard enough to pay rent, let alone shell out 1200-1600 dolars for a couple of new ratings. Get all your training done (and I recommend not stopping till you've got your CFII-MEI) and then start pounding pavement for a CFI job at a school with a few twins. In other words, you might start getting paid to fly now, but it's the ME dual given that's going to really pay off in the long run. So do it all now, while you can still stomach student loans.