Portofino variants

Looking to expand my collection with a white Portofino Chronograph. Can anyone enlighten me about the differences between reference numbers 391009 and 391028?

Thanks!

Portofino 3910-09 has calibre 75320 and was produced from 2011 to 2019

Portofino 3910-28 is a revised version with caliber 79320 and was introduced in 2019.

That is curious, as IWC had utilized the cal 79320 (ETA based) in the even earlier Portofino chronograph, the ref. 3783:

I am not big on the gold, but I love the chrono pushers on this one. It is curious that IWC went from the ETA-based cal 79320 to the Sellita-based cal 75320, and then back again.