Usually it is recommended to mix using studio-speakers. That is what the pro does
'We' do not have that kind of gear that cost 10000 € and up ..Up a lot
Might go at both. Most listeners use headphones, so its good to use it to assume compatibility on that device. And speakers to hear if it is very good on speakers (some songs sound nice at headphones but not on speakers and vice versa) I heard that pros use speakers because of mixing. Deadmau5's studio for example. In rave music they mix songs properly, so the sound engineers know what speaker (eg tweeters, subwoofers) should that freq be positioned during music festivals or concerts. Reasons like the bass doesnt sound messy etc.,
For me, I advise both. headphones to have a very fine/precise rendering, and the speakers afterwards, to validate, with a different rendering, as people will have it while listening in party, in the car or other...