yes, the student loan interest tax deduction is what Im referring to. IBR for me alone would be about $350. If I file with my wife its $1000. A pretty massive difference.
I assume your wife has no loans then, correct? Otherwise her IBR payment would probably be ~$400/month, so it'd only save you about $250/month (which is still a fair amount).
Either way, it sounds like the two of you will have a very good combined income and could absorb that high IBR payment. I still think it's worth considering filing jointly to take advantage of all the things the government "pays for." Like being married (could save you ~2k in taxes), paying student loan interest (you pay $2500, which results in lower taxes AND a lower AGI which then saves you a little bit in IBR the following year), and going to school. The latter could result in you getting a lot of money back--if your school charges you tuition on or after January 1st (during the 22014 tax year) you are eligible for the lifelong learning credit (not to mention you're eligible this year if you file jointly). My school billed me in December, so unfortunately I'm not eligible this year--if I was, I would have gotten all my withholdings back (approx $3000). My wife doesn't make too much though, and I only have half a year's salary to report, but I think it'd still be worth you looking into, because once again I don't believe you can claim that credit unless you file jointly.
My guess is if your wife has no loans then you will still pay more if you file jointly, even if you qualify for all those tax breaks, since your IBR payments differ by almost $8,000/yr. But I think that gap will be bridged pretty well by all the money in taxes you save. Plus, you will have then paid $12,000 of your loans off, compared to $4200. Personally I'd consider paying even more money on top of your IBR payment, so that you have a good chuck paid off by the time residency is finished.
FYI, it's possible with your combined incomes you won't qualify for the student loan interest deduction--it gets phased out at certain incomes.