The M88 has a wider and flatter frequency response and captures more highs and more lows as well as handling transients a little better. That makes it more versatile and a better choice for percussive sources as well as instruments with LF content (kick, bass etc). It has a different sound from the M69 which (IMO) generally works better for vocalists, esp females.

By comparison, the M69 is a bit more mid-heavy and can work in some scenarios where a narrower FR will work or for a slightly more lo-fi or retro effect.