I pay $42/month more in gas, but my payment went down $20, so I went negative $20 a month. However, my GTI was spending more time in the shop than in my garage or on the road, so an extra $20 a month for reliability and for me a more enjoyable drive (plus more options and better stereo) was definitely worth it.
Really sit down and do the math on this to see if it would make sense for you. If $$ is what you want to save, then you gotta take everything into account. Payment cost, insurance cost, fuel cost. If you want, I can give you the formula I used to determine the gas cost/savings of one versus the other.
Mine was not a financial decision, but money always plays a part in life. Yours seems strictly financial, so you'd need to save quite a bit to make it worth it. $10 or $20 per month would be negligible. You need to save well over $100 a month to make it worth it I wold say. But what is worth it for you?