Yes, the more interesting question is BB NC v. FB NC.
The more obvious answer would be, as you state, FB.
For one thing, an IU NC in football would be the most unlikely of any program win since WW2. Dating back before the BCS, which began in '98, there were some claimed championships which look odd today: '52 MSU (1st for MSU); '53 Maryland; '54 UCLA; '58 Iowa; '59 Syracuse; '60 MN and Ole Miss; '64 Arkansas; '76 Pitt; '81 Clemson (1st); '83 Miami (1st); '84 BYU; '90 Colorado & GT; '91 Washington. With the BCS & playoff systems came more clarity and only iconic programs win now. So........an IU football NC would certainly be historic.
But long-term effect?? How long has it been since anyone thought about about Maryland, GT, Syracuse, and Colorado as NC contenders? Similarly, the demographics are just not on IU's side in the long-run. If we DID win a NC in, say '28, would it shock anyone if we were back in the bottom 8 BT programs in '35?
OTOH, it's been 37 years since we won a BB NC........forever, as far as 50% of the population is concerned. BUT, if we DID win another, we'd be back in the top 6 college BB programs in people's minds right away.