I'm not sure about any benefits/negatives of 140mm on the H100i specifically, but I do know about the negatives of 140mm fans on radiators.
140mm fans generally move a lot more air with a lot less noise than the 120mm fans. However, 120mm generally have far more pressure, which is good for radiators and heatsinks.
Noctua makes two fans that would work well with the H100i. The NF-A15
and the NF-F12
. These two fans both use 120mm but the NF-A15 is a 140mmx150mm fan whereas the NF-F12 is a "standard" 120mm. The increased surface area of the NF-A15 will have more air contact with exposed fins, i.e., as long as there are fins that are normally covered by the fan housing, these fins will have air on them with the larger fan. The NF-F12 on the other hand has far more pressure and thus will be better at pushing air though the fan, which means that the fins that are receiving air are getting more air pushed across. Personally, I would prefer more pressure to push air through than more surface area as the increased pressure reduces the effect of the resistance of the fins.
That said, I just looked up a picture of the H100i and I don't think two 140mm fans will sit next to each other on that radiator. Just because the mounting for a fan is 120mm spacing doesn't mean that the fan is a narrow as a 120mm fan.