Are those the only two options? Sometimes you can find a good deal on just two barrel lengths. If so, maybe consider getting the 24" and taking to a smith to cut to an inbetween length. I personally don't see any issue with 20" barrels for 300 yds.
My experience is wetside rainforest and long barrels can suck. It is not the weight, but the length that gives me problems. Bumping trees, low branches, etc.