Both would be acceptable in Czech (and are among the accepted translations on Duo). I personally prefer the instrumental here as it sounds richer to me but you might hear me utter the nominative as well, when I don't think before I speak. :) Not sure when you can use both, but as a rule of thumb, I'd say "to be something of someone" can go with both cases.
To me instrumental sounds a bit too pompous or bombastic or wanting to sound posh. There are some advisory rules for nominative / instrumental that no one remembers. The difference is supposed to be if the "is" is a general or eternal fact or just a temporary fact. I wouldn't bother with these.