I have my septum and I got snakesbites done a few months ago. (Two lip piercings, one on each side of the lower lip)
My only problem with the lips is that the metal of my studs clicks against my teeth sometimes. It shouldn't be a bother with your singing, sound-wise, BUT! Until you are used to the lip piercings and adjust how your lower lip moves, the backing of the jewelry (if you have studs) can hook on your lower teeth. If you make a lot of facial expressions while singing you might snag them until you can adjust. I don't know, but one lip piercing might be easier than getting two done at once. ;]
Anyways! After all of that- I vote for septum. It'd be cute. Use a smaller nose-ring, captive or horseshoe and it will look more dainty and feminine. Mine isss 3/8 I believe. I had a 1/4 and it was too small. That's my personal preference after having one a few years.