One fundamental difference with Legion is that Chapel is explicitly parallel. As I noted in another comment, Legion has sequential semantics [1]. As far as I know there is no subset of Chapel that has sequential semantics while still retaining the ability to execute code in parallel. This means there are classes of bugs that Legion avoids that Chapel cannot.
On the other hand, Chapel lets users write code that is pretty high-level while still retaining the ability to do distributed execution. In particular, Chapel's forall loops can be combined with domain maps to get distributed-memory execution of code that, if not literally sequential, still looks pretty darn nice.
Domain maps in Chapel provide the ability to specify how data is distributed around the machine in a way which is decoupled from e.g. the forall loops over the data. There is a surprising amount of subtlety to this, and I admit I initially underestimated exactly how important domain maps are in Chapel. In particular, domain maps are permitted to make internal copies of remote data, which means that domain maps can be responsible for ghosting/halo exchanges. Because domain maps are user customizable, this essentially permits the user to put their low-level data movement code in the domain map, while keeping the rest of the code clean and high-level.
The closest analog to domain maps in Legion would be partitions. But in many ways they're completely different beasts. Partitions in Legion offer a similar amount of expressiveness, in the sense that the user can specify partitions to contain any subsets of the data. But in Legion, data movement in fully automatic. Legion does not permit, or require, the user to write custom data movement code of any kind, no matter how complicated the partition is. Also, partitions are not fixed to specific locations in the machine but can be dynamically rebalanced. Of course all of this is technically possible in Chapel, because domain maps are user-customizable, but none of it comes out of the box and as far as I know this sort of behavior is underexplored in Chapel in general.
As a final note I'll mention that the Chapel folks are really friendly. I've been doing some comparisons with Chapel recently and I found it relatively easy to get help on issues I was having, and that certainly has not been true of all the systems I've been using!