One problem with FFP is the reticle size changing.
Looking through a Vortex Viper 6-24,
at 6x the reticle was very thin, I could imagine lossing it trying to shoot a large dark object.
And then at 24x the reticle seemed almost too big.
Unless you are using the reticle for hold overs or ranging targets just get SFP as the scopes are generally cheaper.