MIDI is textual descriptions like a book. Letters and numbers. C#5, E4, G3. Instructions that can be read and misinterpreted in any way imaginable. Channel 2, channel pressure 45.
WAV is an audio recording, like an audio book. A recording of someone reading the text out loud or making noises, sounds of birds singing, the reader singing, burping. Sounds you'll have to guess where they came from.
By first recording your music as MIDI, you will be able to edit it like text, read and re-read it, perform it in infinitely many ways, with different instruments if you like. You can send the MIDI file to someone else, and they will make their computers perform it, or even print it out as notation and have a symphony orchestra try and perform the notes and control-change commands.
When you make an audio recording of a performance of your MIDI file, that performance will preserve... one performance of the MIDI file. You can even mix in the sounds of the audience booing at the performance. Every little nuance will be recorded and preserved. But getting the original notes and control-change commands that there were in the original MIDI file won't be possible.
If you have a few kilobytes data storage space to spare, why not keep a MIDI version as well as an audio recording of a performance of it. It may come in handy later, letting you edit it and make recordings of further performances. Or if you intend to be an audio book author, why not keep a textual version of your book as text, in addition to an audio recording of someone or something speaking?