Replying to my own post here just for the sake of the archive should someone else ponder Operation Neptune and turn this up in a search...
First, my original searching skills were lacking. I eventually found a nice AAR of the first CG day over on BGG:
First Play Through of Initial CG Scenario | Operation Neptune
boardgamegeek.com
It handles river crossing in a CG in a much simpler way than the Dinant CG in the French Module.
This is a really good point. There are a lot of additional rules for the crossing in Dinant. Actually, more broadly Dinant just has far more rules overall (nine pages before even getting to CG specific rules).
In addition, the scale of Dinant is significantly larger. The Dinant map is three times the area of Neptune and not surprisingly the initial scenario squad counts are about a factor of three higher as well. In addition Dinant has a lot of AFVs which can of course increase play time.
So I think while one might view this as very similar to Dinant (river crossing, about the same number of CG dates, a night CG date) there is in fact such a significant difference in scale and complexity that these are really not interchangeable in a practical sense.
If looking for a grander scale, longer play time, and chromier experience then probably choose Dinant. If looking for a more manageable OB, lighter time obligation, and very little chrome then probably choose Operation Neptune.
As to my personal conclusion - Noble Knight is having a sale and ON is down to $49 while also pushing the rest of my order across the free shipping threshold, so one of these days (years?) I can hopefully report back on my impressions of playing the CG.