I don’t know of a good online tool to compare. If you buy TurboTax to install on your own computer, you can prepare multiple returns because each is a separate file, like a separate word processing document. If you used TurboTax online, you would need three separate accounts with different usernames and passwords, to prepare a joint and two separate returns.
@xmasbaby0 might have a more comprehensive list of the items that are not allowed when you file jointly. Off the top of my head, you can’t claim the dependent care credit for putting a child in daycare, and if you want to use an FSA from your workplace to pay for child care, the limit is $2500 instead of $5000, and only one parent can claim the child as a dependent. Your ability to contribute to an IRA is also limited when you file separately.
The problem with focusing your thinking on IBR only is that you are guaranteed to pay higher taxes for as long as you file separately. Whether you save more in interest payments over the long run depends on interest rates, possible loan forgiveness, and whether a forgiven loan is taxable. If you plan to qualify for PSLF, you may have to file separately for 10 years to benefit. If you want ordinary loan forgiveness, you may have to file separately for 15 or 20 or 25 years — the laws keep changing and I haven’t kept up with the changes. Whether IBR really saves you more money in the long run will depend not only on the tax benefits that you lose but also on future changes to tax law and student loan law, which is pretty unpredictable at this point.
I can tell you that I was on a 27 year repayment plan, which I finally managed to pay off 4 years early, but it was a real drag to be paying my student loans after my daughter graduated from college. Of course, I was not eligible for any loan forgiveness programs back then, whether created by Congress or waived up by the president‘s magic wand (which may or may not survive court challenges).
