Artwork, D&D, Gaming, and Transness. She/Her
Takes a deep breath...
Ah, glorious Terf-free webspace. I like it already.
I think one shots are better for teaching new players rules and mechanics because, and this is based off my experience, one shots tend to be more wild and out-there mechanically because its a time to just let loose and have fun with usually more obscure or unused mechanics there's no real characterization for anyone or anything so having a new player let loose in a one shot would be a great learning opportunity. They can just go! Have fun! Destroy shit or whatever!
I don't think this holds true for teaching them about narrative though, for obvious reasons, unless the one shot is supposed to be closer to a longer campaign in narrative and its been pre-determined thay everyone won't go ham, but again, this is based on my one-shot experience.
I GMed a one-shot last month that was easter themed. Had my players rescue the easter bunny while battling cockatrice-chicks and chocolate golems.
I used that oppertunity to introduce one of my friends to 5e online, as he played once a month irl but wanted to play more and hadn't used roll20 by that point. It was a great time, and a great chance to introduce a friend to my other friends.