Look closely at the first concertina at the link below and you will note BOTH doomed and flat buttons. The doomed are C and E. Everything else are flat buttons. The reason that I did it this way is to have a system help me know where I am on the keyboard. (the concertina shown is the one I had purchased and use).
Therefore, every song I play, day after day, year after year, I am using both. So what is the difference - not much !! But there are some slight advantages to both.
Rounded: when I have a finger on one button, but am going to need that same finger play another note (but not interrupt the sound playing), I will need to quickly replace the finger with another. A rounded key makes this transition easier.
Flat: When it is hot out, and one's hands begin to get a bit sweaty, the moistened fingers can more easily slip off of rounded keys than flat keys.
Regarding using both way I did in the image shown, this has been minimally useful for navigation purpses, but I do think it looks nice. And I do suspect, if I really trained myself, it could be much more useful as a navigation tool.
If I were FORCED to choose one over the other, I would probably go with flat keys as my fingers will be more apt to on the keys and not slip off.
Hope this helps.