It depends on what you're trying to achieve. CV/Gate can send notes to a synth, or be used as modulators or control. MIDI can do the same, but digitally.
If you have an old analog synth without MIDI but with CV/Gate, use CV/Gate.
If you have a synth with only MIDI, use MIDI.
If you want to send polyphonic notes to a polysynth, use MIDI.
If you have a synth with both, you could use either, but CV/Gate only gives you mono notes, so MIDI is better if you need poly.
If you want to send more controllers than you have CV outs, use MIDI.
If you want to control multiple synths, use MIDI (unless you have enough CV/Gates to go around).
If you have modular/semi modular gear, you could use CV for all sorts of things besides triggering notes. You could use MIDI to sequence the notes and CV to modulate the filter cutoff, for example.
EDIT: After searching your other posts, sounds like you have a bunch of semi modular gear already, so you can put those CV outs to good use. Be creative. You don't have to use it for notes if you also have MIDI, you can use it for all sorts of other things such as modulation... for example, sequencing filter cutoff on your Grandmother or Neutron.