Numbers (including number based puzzles) are naturally easier for a lot of people to process and sort than letters, so it is possible a U could have a slightly worse effect than a 2.
If you give people the same or similar puzzles, there's a good chance the majority will find the one that uses numbers easier than the one that uses letters
--
Never underestimate the power of stupidity in numbers. ~War13104
Never stoop to the level of idiots. They will drag you down and beat you with experience.