One way to compare determinism and fatalism would be by tense:
- According to determinism, all future events will be determined.
- According to fatalism, all future events are already determined.
This might look vaporous (and it mostly is), although questions about the existence, or nonexistence, of the past, the present, or the future, or distinctions between tensed and untensed temporal logic, etc. reveal a subtle wealth of mystery in this vein.
As far as free will goes, if we are speaking of libertarian free will, then on some accounts it is still determinism that is more problematic (because determination by physical causation is more well-grounded than determination by logical necessity in general, perhaps), whereas from other angles fatalism might sound more troubling. For example, there's probably a way to read Kant's thesis that free will exists alongside, and not in absolute conflict with, spatiotemporal causation, such that Kant might've taken (theistic) fatalism as more at odds with his interpretation of free will:
If existence in time is a mere sensible mode of representation belonging to thinking beings in the world and consequently does not apply to them as things in themselves, then the creation of these beings is a creation of things in themselves, since the notion of creation does not belong to the sensible form of representation of existence or to causality, but can only be referred to noumena. Consequently, when I say of beings in the world of sense that they are created, I so far regard them as noumena. As it would be a contradiction, therefore, to say that God is a creator of appearances, so also it is a contradiction to say that as creator He is the cause of actions in the world of sense, and therefore as appearances, although He is the cause of the existence of the acting beings (which are noumena). If now it is possible to affirm freedom in spite of the natural mechanism of actions as appearances (by regarding existence in time as something that belongs only to appearances, not to things in themselves), then the circumstance that the acting beings are creatures cannot make the slightest difference, since creation concerns their supersensible and not their sensible existence, and, therefore, cannot be regarded as the determining principle of the appearances. It would be quite different if the beings in the world as things in themselves existed in time, since in that case the creator of substance would be at the same time the author of the whole mechanism of this substance.
And yet others might recoil equally from determinism and fatalism together; the degree and angle of recoil varies with one's sense of how libertarian possibilities obtain. E.g., a believer in future-tense determination might think that "all spacetime as a whole" is not an admissible term, or is an inherently underdeterminate term, so that when a local force causes something going forward in space and time, the causation is strict, whereas when the universe "as a whole" behaves in a causal manner, there is a necessarily underdeterminate accompaniment as far as effects go; and so one might imagine that indeterministic free will involves the causal activity of the entire universe (taken as an individual object that is not absolutely reducible to the sum of its contents).
Recommended reading (from the SEP (list is not exhaustive)):
- Temporal Logic
- Causal Determinism
- Foreknowledge and Free Will
- The Metaphysics of Causation
- Arguments for Incompatibilism