There are no words to describe the thrill of her voice at its best. The "Casta Diva" is a revelation and not to take a back seat to Callas, at all. Where Maria is clearly superior (IMO) is in the softer more introspective moments such as the act 3 opening "Teneri FIgli" etc.And through most of act 4 they are a draw for my money. "Ah non Tremare" is clearly Callas territory, but Cerquettiis no slouch here.
Vespri brings a different set of requirements and here Cerquetti is even more a match for Maria than in the Norma. THIS is as good as you'll ever hear. Kept me up very late last night.