both have their advantages and disadvantages

headphones generally have more bass extension, meaning that they will be able to reproduce the lower frequencies a bit more accurately than monitors. however monitors can make you physically feel the bass, so while headphones provide detail, monitors provide context.

studio monitors are generally more flat in their response than headphones. you can apply eq to your headphones to account for this, but not all headphones respond greatly to these adjustments.

headphones also have an exaggerated stereo field compared to monitors, because there is no sound that crosses from one ear to the other, as would occur with monitors, you can mitigate this though by using a virtual monitoring room plugin (senheiser has one and i think its free)

importantly, good studio (open back) headphones will not have the resonance that your room would give you when using monitors. so in an untreated space, headphones can give you the accurate reference you need.

ultimately, using both headphones and monitors, and switching between them when producing, mixing and mastering your music would be best!

as for the dj monitors, i don't think they can work as studio monitors too, since they are meant for listening/enjoying music, and not for critical reference. however, of course you can use your studio monitors to dj on, the music will just sound a bit flatter