I'm a native speaker and I would say that the meanings of the two are identical but that the "would" construction is a little more formal, or poetic while the "used to" construction is more commonplace.

If a word starts with a vowel SOUND (not a vowel) then use an. Several consonants in English have names that start with vowel sounds: F (eff), H (aitch), L (ell), M (em), N (en), R (arr), S (ess), and X (eks). As someone has already noticed, the name of the vowel U (yu) starts with a consonant, so it is "a U". Words beginning in U but starting with the Y-sound also take a: a uniform, a university, but an umbrella.

In English an abbreviation might be pronounced like a word (RADAR, NASA) in which case it is called an acronym. It might be pronounced as a group of letters (SMS, DVD) in which case it is called an initialism.

So the article for the same abbreviation may change depending on how the person writing it pronounces it!