No, you encounter Chief Irons with Claire in both. In Scenario A, the G pops out of him, in Scenario B, he's pulled down into the sewers and ripped in half by Birkin.Same, i can't really decide so i just cast my vote LeonA/ClaireB. I believe that's the way i played it first.
Playing Claire A almost makes sense because only in the A scenario do you actually talk to Marvin, in which he brings up Chris and S.T.A.R.S., which would make sense for Claire to be A. Then again i almost preferred Leon fighting William Birkin and G with Claire dealing with Mr. X. Also don't you only encounter Chief Irons on the B scenario?
Either way i like them equally.