Quality of resin ND filters?
 

So far, I've been using my FS100 either with the Heliopan VND filter, or with my hefty RR mattebox and 4x5.65" ND glass filters. Now that I've bought a smaller/lighter mattebox (MB-600; nice), I'm after 4x4 filters. Never used anything other than glass filters, so have no experience on the picture quality using resin filters (and perhaps stacking 2 such filters). But the fact is that for a price of a single Formatt 4x4 ND glass filters, I can buy 3 Hitech 100 ND filters, so I'm a bit lost...

I'd be grateful for sharing your opinion on the quality of resin filters in general, and the Hitech brand in particular. Also, will a typical 4x4" filter tray accept "system 100" (100x100mm) filters?