I have two Hastings slug barrels. One on my Wife's 11/87 the other on my Beretta 390, both are 24 inch models. The 20 inch was not available. I do not think that you will notice a big performance difference between the two lengths.
The warning on Hastings barrels is for solid copper slugs not copper jacketed slugs. The warning is specifically for the early model Remington solid copper slugs. The warning states that the new model Remington slugs are fine. The problem was not with the Hastings barrel it was the Remington slugs! Hastings was not the only one to have trouble. They were the only one to put a warning about them on their barrel. Remington made changes to their slug which have apparently fixed the problem. I happen to use the Remington copper solid all the time in both of our Hastings barrels!
Good luck,
Tuco