Running a 100/120mm on a 29er fork. Is there likely to be any real world difference in the stiffness?

Ride wise I’ll be riding just cross county stuff, nothing over the top gnarly and if it does get that way I’ll just slow down a bit anyway.

EDITED: As I was on the phone and looking back at what I typed was nonsense.

I doubt there is much difference for that sort of use but 9mm qr is generally getting harder to find in forks so 15mm is the way to go for future proofing (until the next ‘standard’ arrives) if that matters to you.

Doesn’t bother me too much hamish as running Hope hubs and have both options. Looking at the CRC forks there is a lot of nice choices on the 9mm side of things.

I doubt there is much difference for that sort

You will notice a difference, especially if you’ve an older QR fork and move to something newer/stiffer etc.