Once you get your nice new regional job, the MEI doesn't disappear. Instead of building mindless time that will blend in to your 50,000 multi when you retire as an A-380 captain, why not get an additional rating that makes you marketable, different, and has potential to offer job possibilities down the road, say if you should get furloughed. Thankfully since we are in such a good hiring boom, your furlough will probably never happen.
While getting your MEI you are building multi time. Get it at a place where it is cheaper and if you have extra cash laying around go build some more. You need 15 PIC to get the rating (which training can count towards).
Easy choice dude.