Obviously you know that I think money is important from the other thread, but if the schools are equal in rank and you feel approximately the same about each program then take the money IMO. You're talking about almost 100% difference in salary and that's huge. You'll have almost 75K more after 5 years, that could be used to invest, pay off student loans if you have them, by a house, etc. These things may not seem important right now, but they will be in the future, so why not get a head start?
30K a year is high roller territory for grad students
As mobytish says, cost of living should also be taken into account, but I don't think it would have such a huge effect that 15K in city A = 30K in city B, but I could be wrong.
In my 2 years as a grad student, I had the opportunity each semester to take on an extra section as a TA to increase my salary by 50%, and I did it each semester except the first. Being poor sucks.
Edit: Also, it's not greedy to take the offer with more money. It's just being prudent. Your time is worth money and the more you are compensated the better your life will be (from a utilitarian point of view) both because you will be more financially secure and you will have more time to do the things you want. Our society is driven by money, and while that might suck, it isn't changing any time soon and generally the more you have the more you are empowered to control your own life (with some exceptions obviously).