Use the fmc! here is why;
CFP
It might have been generated for a different weight. there might have been a last minute zfw of up to 10 tonnes (that's for my 777). Or the ZFW may have increased. In the odd scenario winds may be different then the FP (not very often)
FMC
Optimum = current weight and cost index
Maximum = current weight, coat index and temp
Recommend = current weight, cost index, temp and winds. (uses 100% of fms wind for actual position and a 50/50 mix 200nm ahead of the aero plane with forecast winds).
So in a nutshell CFP for planning, FMC for operational decision making.