I’m not sure how to respond because I’m not sure how you want to use either of them. C Sif hasn’t been great on defense for me. Haven’t played with the configuration much but not impressed with her on D. Offensively sif is sick, I use her quite a bit. My gut says go for sif since you don’t have a holy riposte. Matilda is super cool but I believe she requires more synergy than sif. So with limited info I’d go with sif.
On offense I mainly use Kullervo and I won’t change my main offense team (rainbow) in favor to neither (sif or Matilda).
I will mainly use cSif/Matilda in alliance war (in one of the six teams) and maybe for tournaments.
As well will neither be in main def team since Kullervo is doing great there and as you said cSif doesn’t do great in defense (especially against El Naddaha).