Cybex are such buggers for this, they have so many identical looking seats and no comparison table on their site.
I know that the S-fix is the premium model, one below the Z-fix, so it has all the side impact features and the reclinable headrest and very good protection. It just doesn't transform outwards (get wider) like the Z-fix does.
Looks like the G-fix is a newer generation shaped shell (doesn't necessarily mean anything). The hooks for the lap belt are a slightly different shape, perhaps they hold it in place better than the old shape, particularly when reclined - this can be a problem with typical high backed boosters.
Looks like the main difference is the new recline mechanism.
In general, with high backed boosters, if you have the option I would definitely recommend setting your budget, then take your child to a shop and try everything within that budget range and see how it seems to fit them and how comfortable they find it. That's better than buying online, the features are all much of a muchness and the comfort factors are subjective.