I've worked on an android/ios game and a pc/mac game.
a few things to help you decide:
MIDI and MOD(also XM/IT for some trackers) files are sample based (MOD/XM/IT files use your own samples, MIDI uses the samples provided by the devices MIDI implementation), they are both fairly easy to modify at runtime. You can for example change the speed at which the music or parts of the music (such as the drum track) plays without also changing the pitch or you can just change the pitch of a track or a note without the speed being affected. One set of music tends to consist of several tracks played in parallell (normally each instrument has its own track or set of tracks), MIDI is far smaller than MOD as it doesn't contain its own samples(They're provided by the MIDI implementation(the soundcard or soundcard driver provides them). the quality for both formats depend on the sample quality. For dynamic instrumental music these formats are pretty darn awesome (For vocals they kinda suck, unless you are sampling vocals)
WAV and FLAC are lossless formats and their quality primarily depends on the samplerate and size. (FLAC is compressed so for actually deploying the music it is preferable over WAV)
a CD quality wav file (2 channels, 44.1Khz, 16bit) eats up 10MB / minute) , FLAC slightly less, these formats basically store a digital version of the soundwaves sampled at a fixed rate(1hz-4.3Ghz for Wav, 1Hz-655Khz for FLAC) which makes them extremely hard to modify in a sane way at runtime.
Mp3, Vorbis(ogg/oga), etc are similar to wav files but have been filtered and compressed using a lossy filter/compression, those formats give you a pretty good tradeoff between sound quality and filesize.
[size="1"]I don't suffer from insanity, I'm enjoying every minute of it.
The voices in my head may not be real, but they have some good ideas!