That's why we can talk about musical things in abstract. If you have a [one] [five] [six]m [four] chord progression, it's the same musical statement regardless of whether it's in E major (E, B, C♯m, A) or in B♭ major (B♭, F, Gm, E♭).
It wasn't always this way. Scales used to sound significantly different in different keys. Then J.S. Bach came riding into town with his "well-tempered" scale (also called equal temperament or EDO) and made all the semitones the same size. (Read the actual story of what happened.)
Today there are only a few concerns that would make you pick one key instead of another:
- Singers care about what key a song is in. Singers have a very limited range of notes at their command compared to any musical instruments. Always set your song in a key that puts the melody in a good range for the singer. If you switch singers, be prepared to switch keys. If you're singing the song yourself, don't be lazy: put the song in your best key, even if it means having to relearn the chords. You won't always happen to compose in the perfect key, so just plan on potentially transposing after the melody is finished.
- Musical instruments have their idiosyncrasies that can make one key sound different from another. For example, a guitarist can use a lot of open-string chords in D major, but can't use any at all in E♭ major. The different key might result in a different sound quality for the guitar.
- You don't want to play too many songs in a row in the same key. Even if listeners aren't consciously aware of it, they get fatigued from hearing the same notes and chords over and over. Arbitrarily switching to a different key can help keep things sounding fresh.
- If you're writing a song about someone named Carla, use the key of C, because Carla starts with C. That's what I would do, anyway! And I'm sure J.S. Bach would agree. He loved to work that kind of hidden musical message into his compositions.
Tuesday is Beastly Fundamentals Day.