I'm no expert, but I would think that sweet potatoes have different levels (less) starch than a regular potatoes and that may influence the consistency and feel of the kugel. You could try making it 1/2 each and if it works well, decrease the potato portion the next time you bake the stuff.
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------------------- Mild Mannered Minion ------------------- I feel the pull on the rope, let me off at the rainbow -Anyway, Genesis