Benefits of SFP
Cheaper
Greater availability (many scopes not made in FFP)
Finer stadia lines at max magnification cover less of the target for greater precision in aiming (with some reticles) in some situations
Less complexity, less to go wrong, and greater light transmission since SFP requires 2 fewer lenses than FFP; all of these are theoretical benefits as I am not aware of any statistic proving any of these ever manifest in the real world
Benefits of FFP
Reticle subtensions accurate regardless of magnification; the shooter sets the magnification to whatever is best for a given situation and can always access the features the reticle offers, like ranging, holdovers, gauging amount required for second shot corrections that will be dialed in (assuming matching turrets, another must have feature IMHO), etc. without requiring any additional math to correct for the magnification not being set to the "reference" magnification like an SFP scope necessitates
Some reticles have designs (floating dot, thin stadia at the crosshair) that decrease the FFP issue of the reticle covering too much of the target at max magnification.
I always opt for FFP for any scope over 8x; to me its practical benefits outweigh the potential downsides.
Joe