Well, there are a lot of considerations. To be honest if you're overly budget concious I'm not sure the challenger is the vehicle for you. There are probably more fuel efficient less expensive vehicles that you can purchase.
THAT BEING SAID:
I faced the same decision. v6 should still get 0-60 in 6.5ish seconds. It's quick, it'll get you there. The combined driving mpg on a v8 is supposedly 19 MPG. I believe for the v6 it is... 21? If you drive 1100 miles a month (as I do) you'd buy about 58 gallons of gas with the v8. The v6 would buy only 52. So Roughly 6 gallons, at your local gas price is: 20-25 $ a month. If your budget is dependent on 20-25 $ a month, again I suggest as before.
I drove them both. I loved the R/T. I Like having that power, although, you rarely get to use it. You can only drive as fast as the person in front of you.
Food for thought: The price difference between the most basic SXT and R/T: 6-7k.
You should be able to right now grab a 2012 SXT for 22k, and a base R/T for 28-29k.
Look at the resale vaule on previous models in the same: Are they 6-7k difference? (I have NOT looked)
Another point: I couldn't decide at first, i started looking last year actually, i drove mine home this friday past, and in that time I paid off 3 long term credit card payments, and saved up some extra $ to put down with out having a payment (truck was paid off that i traded in) and shopped around for a month to get a great deal. My payments are in the SXT range, because by the time I left with my R/T Classic I only financed 19k. (And it was loaded)
I myself had to have all the bells and whistles, but I do sometimes wonder if I'd of done better with the v6. I could of saved that cash i put out of pocket and had still less of a loan. My insurance would be less of course too. But again: if your budget is that tight...
For me it wasn't, but I did have a budgeted car payment, i had planned in, which is how i worked it into my budget. With lots of planning.
Drive them both, although be prepared, I loved the R/T from afar for a long time, and debated several times over the past year to just go buy a SXT and enjoy it. But I couldn't settle for a SXT base, i wanted leather, nav and what not, and by the time I got all the options, I might as well buy once and cry once to get the perfect one I would build. So I got mine exactly as i wanted no compromise
The way i always phrase it is a v6 pony car is kinda a teenage girls car. But it's still respectable, and its still a challenger, and TBH: Only car guys would know the difference.