I love starting campaigns at first level, but for me a one of is different i would go with level 3 characters for all. While I've never run a one off for new players, I still think many would enjoy a fuller set of options without being overwhelmed and even if they miss stuff you can help them out and dont have to run deadly encounters against them. You want them to have fun and want to play again. Somewhat experienced players are fine with level 1 starts when they know it begins a campaign, but for a one off people like to have enough interest out of the gate, not just squishing goblins and suchlike (for experienced players, i run around level 4 give or take, for a one-off).
Really though, it depends on if you have any idea ahead of time how many newbies you will get, and if they might get the chance to play those PCs again, or if it genuinely is throw-away. Players love continuity.