When labeling chords in a song, should you use numbers or letters? In my opinion, the answer depends on a few things.
Here are the advantages I see for each option:
- Allow you to easily see what is happening functionally (the movement between the chords).
- Makes transposing easier. Simply play the same numbers in a new key.
- Is the standard in many recording studio settings. In fact, I was in the recording studio today and only one of the musicians in the room was used to letters rather than numbers (and he was getting teased for it).
- Easier to play because you do not have to translate the number to an actual chord. In other words, if you see C minor, you just play C minor. But if you are in the key of Bb and you see the 2 chord, you have to in your mind figure out that the 2 chord in the key of Bb is C minor.
- Easier when the harmony is very complicated. It is just much easier to play letters when you have some strange harmonic things going on. Rather than having #4dim and b6 dominants, it might be simpler to just notate them with letters.
In general, try to get comfortable with both options. Numbers are harder at first, but in the long run, numbers are better for the functional pianist. That is especially true because of the need to transpose. Musicians who transpose well almost always think in terms of numbers.