While I think Haskell is a superior language is most respects and I would definitely prefer it for my personal use, I think SML is probably a better language for teaching, particularly for a language-theory course.
Two are two particular issues: laziness and the type system. Laziness is both complicated in practice and makes reasoning about the evaluation semantics more challenging. It can also lead to unexpected behavior for those unaccustomed to it (which will be nearly everybody). Furthermore, the type system is much more complex which makes it a bad model for learning about implicit typing and type inference.
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While I think Haskell is a superior language is most respects and I would definitely prefer it for my personal use, I think SML is probably a better language for teaching, particularly for a language-theory course.
Two are two particular issues: laziness and the type system. Laziness is both complicated in practice and makes reasoning about the evaluation semantics more challenging. It can also lead to unexpected behavior for those unaccustomed to it (which will be nearly everybody). Furthermore, the type system is much more complex which makes it a bad model for learning about implicit typing and type inference.