First, the mounting systems of both masks differ, and their inside-dome receivers are shaped accordingly.
Inside dome of Rubie's Fiberglass Limited Edition (FG LE):
Inside dome of Don Post Deluxe (DP DLX):
The paint job on the Rubie's - in particular the metallic gray - was not as nicely done as the Don Post Deluxe's. The metallic gray on the Rubie's seems flatter, and duller. Although not completely screen accurate, the Don Post Deluxe's metallic gray was darker but the paint job was done more elegantly, and thus how both helmets appeared without flash photography was notably different.
The Rubie's just looks flat:
The DP DLX:
Now other differences are a bit more subtle. I don't want to fully describe the tells because unscrupulous people may want to repaint and modify a Rubie's and try to pass it off as a Don Post Deluxe, or simply mis-advertise a Rubie's as a DP DLX in their eBay auctions. Such tactics wouldn't really fetch more money in sales, as a Rubie's MSRP is equal to or higher than an old DP Deluxe - and there are a variety of DP Deluxe recasts out there.
But the Rubie's looks a bit more "tired", and details appear less defined. Perhaps it's due to the mold being tired (unless we're talking a plaster original because I really don't see a single silicone negative mold surviving 1,000 pulls. So whether this "tiredness" is due to modification (to distinguish it from the original DP DLX run) or some other factor, who knows. But the Rubie's FG LE looks a bit more beat up, and does not capture the DP DLX's elegance:
Nevertheless, I feel the Rubie's FG LE has some promise if you feel bold enough to strip the paint and repaint it.