The only part of Fizban's that seen play so far are the Dragonborn variants, because my two oldest kids exclusively play Dragonborn.
The book has earned a high place in my esteem for my oldest's use of it as an art refefvook to fill reams of Notebooks with fresh dragon designs.
We like Dragons in these parts.
Also, it has the 5E version of the objectively best D&D Monster, the Dragonne, so it's pure S tier territory, forget lowly A+.
As to Bigby's...I like Giants, amd Giants are more, how you say...expendable...than Dragons are, so I think theybwill se some use. Like Dragona, Giants can also be pretty fleshed out NPCs, so James Wyatt's extra flavor material and maps (because he loves tables and maps, I'm sure they will be present) will be appreciated.