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
Nobody had a problem figuring out that the iPhone 4S was the hot new thing they wanted.
Is S better than U?
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SuperNiceDog didn't have to reconcile his name...
But Dauntless Hunter is now MrGreenonion