I don't know enough about Lacazette to make an opinion, but he does seem quite highly rated - and I would say, given the style difference, it is probably Griezemann keeping him out of the french side rather than Giroud.
They picked Gignac over him. Hardly a ringing endorsement.
Gignac - 77 goals in 119 appearances for Marseille/Tigres over the last 3 season
Lacazette - 76 goals in 138 appearances for Lyon over the last 3 season.
Giroud - 65 goals in 141 appearances for Arsenal over the last 3 season.
Gignac is no mug.