This is my opinion,
Right now I am cleaning up and getting my music library in perfect shape. I have downloaded open ID3 library from Github and modified to match what I need. I have done extended research on 2.3 vs 2.4 just because I am detail freak, I would like to capture every bit of the detail. That's why I have compiled my own version of ID3 library to save those tags.
After doing more than 4 months of research two years ago, I have settled on 2.3. I know it is missing some very good information but I have not found player or cloud player that will take advantage of those tag. They all are settled on 2.3 and even in that they do not support all of the specification. I think, in part it is fault of ID3v2 specification because it is not "standardized" by big "boss" and there are not any big backers either. Just take iPod and iTunes.
So I would say stick with 2.3 and more player will support your tags than 2.4.
Though, for a while I will be adding few (about 8-9) custom frames in 2.3 to store my detailed information.
It just my opinion.
But it would not hurt to allow user to select which version they want to keep. So I would say support both 2.3 & 2.4 and then let user decide what they want to use.
