trying to learn here, what would the difference in performance be in a 24-26" 300 wsm versus a 20" 300 prc with the same bullet weight?
The 300 PRC has a higher volume case, a faster standard twist, and greater head height (how far the bullet protrudes from the case). This means it can shoot heavier bullets and bullets with longer ogives (and consequently higher BC's). This would make a difference at long range. The 300 WSM is limited by the short action (2.86") and slower standard twist rate.*
If you want a direct comparison using the same bullets:
I looked up data for the 212 ELD-X for both on Hornady's site.** When both have 24" barrels, the 300 PRC has a max velocity of 2898fps and the 300 WSM is 108 fps slower. With that particular bullet, is likely that a 300 PRC with a 20" barrel would be virtually the same velocity as the 300 WSM with a 24" barrel. The 300 PRC would use 81.1 grains of Staball HD and the WSM would use 61.7 grains of Hybrid 100V.
With a 180 grain Nosler E-Tip, the 300 PRC's velocity is 3190 fps and the 300 WSM is 171 fps slower when both use a 24" barrel. This time, a 20" 300 PRC might have a bit of a speed advantage.
* It should be noted that the 300 WSM can be used in a longer action and a custom barrel with custom chamber fitted. That, of course will usually cost more than a factory 300 PRC rifle. Handloaded ammo would be needed to take advantage of the custom rifle. This setup would likely increase the WSM's velocity and possibly put it ahead of the PRC in short barrels.
** The 212 gr ELD-X would have a long jump to the lands for the 300 WSM. It might have accuracy problems. The heaviest 300 WSM factory ammo I could find was 200 grains.