a casa signorile is usually definded as a mansion, or an exclusive/luxury home, whereas 'di pregio', means 'di qualità, as you can see below from the De Mauro's definition, and here in Australia we simply call them 'quality homes' (I've been house hunting for a few months...)
signorile in Italian, means 'di lusso', but also sort of 'storica', you know those 'antique' villas, which used to belong to Lords (like some famous ones in Tuscany), and since they make a clear definition, as you said, I'd go for 'mansion' for signorile, which is a luxury home, but could also include the 'Lords villas'.

DeMauro:
di pregio loc.agg.inv. CO di particolare valore o qualità: un vino di p., un oggetto di p.

I think my description's got a bit confusing at the end,but hope I've given you an idea...