How much of a difference does a head stamp make?

[SippyCup]

For what you’re looking to do mixed will be fine. I wouldn’t be running up near max pressure though. Like other have said, if you want to eliminate one thing from the potential issues of developing a good load you’d want to run same brand and lot.
If you keep pressures low 200-300 piece of quality brass will last the life of the barrel of both the calibers you listed.