I used to have an ED80 Pro, and currently have the Megrez 90, which is the next model up from the ZenithStar 80 in WO's current product offerings. I sold the ED80 when I realised that the combined cost of the scope plus the 10:1 Crayford focuser upgrade I had on order was more than the Megrez 90 was going to cost me, and I've always liked the look of the WO line.
First, on the diagonal. To be honest I don't think many people will really see a difference between a dielectric and regular diagonal. That said, I do own a GSO dielectric myself. It is only a few percent brighter than a non-dielectric from my understanding. According to
http://www.cloudynights.com/item.php?item_id=82 , the key difference is that the coatings on the dielectric diagonal will last longer.
Regarding FPL-51 vs FPL-53, -53 is newer and more expensive and allows for easier colour correction design, but is not automatically better, as lens design still plays into this. I think from reading up on the math, in an 80mm refractor they are not going to have much detectable difference, unlike they would in a 110mm or larger. However, if you are planning IR work, FPL-53 would be to your advantage as it allows correction to continue out farther from the optimised wavelength.
Having had an ED80 Pro and my current WO Megrez 90, I can say that while the ED80 was not poorly made, the WO build quality and 10:1 Crayford focuser do give a significant advantage to the WO. The 10:1 microfocuser is something you don't get on the ED80 unless you buy an upgrade, and it is very nice to have.
The ZenithStar 80 is also a faster scope than the ED80, at f/6.8 vs f/7.5, so while it will provide slightly less focal length it will be somewhat faster photographically and also have a wider field of view.
Regards,
Eric