The Grad assistant thing seems like a once in a lifetime opportunity. Never in your life will you ever have so much free time and at the same time be able to live comfortably. Once you begin a 9-5, 5 days a week job or similar, you're generally stuck there until you retire, win the lottery, get promoted to such a ridiculously high level that you can either retire early or work part-time, or die.
Plus you've had 3 years at the place you're working. Whose to say they or a similar company won't take you back after you've finished your MBA? Plus if money is tight you could try to find a job that does morning/evening shifts a couple of times a week (if that's readily available to you) if you need some extra dough.
Is there anyone at your current job you can talk to about this without seeming like a traitor? If you're close to a manager or someone in HR or similar, ask them whether doing the GA will make them more likely to hire you again in the future? If you're not planning on going back there though then don't bother.
I have a feeling if you pass up the GA thing you will constantly regret it. I say go for it.