S-Log and 422 10 bit recording will just give you much more flexibility when doing a serious color grade. If you adjust PPs and exposure carefully to come close to your intended final look, the FS100 can come quite close. It will just leave much tighter margin for error.
i have tested them side by side... the project we did the tests for was pointed at 422/YUV broadcast deliverables, and i agree with Nomad, less room for error on the FS, but close enough with a 422/YUV pipeline that using an FS as a second unit/C cam for a F3 shoot seemed reasonable.
A MUCH bigger diffrence when taking the post into a 444/P3 pipeline with DCP deliverables at the end... with the F3 in S-log & gradeing in P3? The FS is really not in the same ballpark...
So in our tests it really depends upon where & how the images going to be shown, can't make a blanket statment...
The 8bit-ness is not s huge deal on most shots, as long as you have exposures & PP's dialed in.. and where it was an issue we were able to nullify them to an acceptable dull roar with an aggressive post treatment; masks, trackers, anti-banding, de-noise, de-grain/re-grain, sky replacement in one shot...
Starts to make the F3 look cost effective by the time all the bills are added up...
My father used to say "I is not what you paid for it, it's what it costs you that counts"
d
PS... for my own low bud film, i bought a FS rather than rent a F3, Alexa or RED, and i have a ton of experience with timeing f900's, Viper's, 5/7D's Alexa & RED... (and film)