21" youth barrel vs 26" standard, I like shorter barrels.
A big plus for me was the shorter LOP. I'm 6' 230lbs with long arms, no neck and a 50" chest so when seated against a tree the short stock allows me a full range of motion without getting in a bind as a full length stock could do. It was unreal how much more comfortable the youth stock was.
If you already had a 20 gauge build off it, but I went with the youth for the above reasons.