I’m looking to buy some new forks but am currently running 9mm qr is there any point moving up to 15mm does it
Make the fork any stiffer or and better for braking

I don’t think it makes any difference QR – 15mm, but QR to 20mm is noticeable.

If your current wheels have changeable adaptors go for it. I’ve had 20mm, 9mm and now 15mm. The 15 and 20 feel very similar in terms of improved control over the 9mm.
If you can’t change your hub adaptors you’ll need to factor in a new wheel/re-build to the cost..

20mm front always. even on my rigid forks

Depends on how hard you ride, if you’ll notice the difference.

With DC forks, the jump from QR to 20mm (with 203mm rotor) was night and day.

XC forks with 120mm travel, and 160mm disc? Less noticeable.

Using 185 rotors