Many of my students had this dilemma in the past as well, and I advised the same way- get everything you can as soon as you can. Once you become an instructor, unless your lucky enough to find one of the few relatively well paying jobs, you’re going to be digging up change from in between the car seats just to put gas in your car to get to work. The last thing your going to want to do is spend more money on ratings, and it's going to get put off for eternity. Eventually when you do go for that rating, it'll be more expensive, as one thing we can be guaranteed of in this world other then death and taxes is that the cost of flight training only keeps going up and up.
Another thing to consider....the longer you don't have that Multi rating, the more lost opportunities your going to have to build multi time. You'll be amazed at how quickly you'll network as a CFI, and soon offers and opportunities to fly multis will be coming in.
Just my 2 cents.