This is a tough one to answer. The Morning Star is a completely different hotel that is down a rather large hill from the Frenchman's resort. You'll either need to hoof it up the very large stairs to get to the Resort or take the shuttle that runs between the two properties.
The main hotel is where all of the action is but by the same token, the Morning Star comprises of separate buildings, sort of like villas, so privacy is guaranteed. The Morning Star is also right on the beach (and I mean right on the beach) and that may be important to you.
You didn't mention how many days that $300 upgrade fee covers. If it's a full week then I'd say it's probably worth it, but for a weekend I'd probably opt to use the money on other things.