Q: Should I monitor on headphones or loudspeakers?
The problem with monitoring on headphones is that it is easy enough to create a mix that sounds good on headphones, but that mix might not also sound good on loudspeakers.
If a mix sounds good on speakers, then it almost certainly will sound good on headphones too.
In years gone by in audio, it used to be thought that you should mix using a really good pair of speakers, which would be big so that they could reproduce the bass end of the music well.
This would be done in the assumption that even if the listener didn't have a really good pair of hifi speakers at home, that is what they would aspire to own.
So if a mix created on really good speakers didn't sound good in the listener's home, then it was the fault of the listener's speakers, not the mix.
Things have changed however and people now listen on all kinds of speakers, headphones and earphones.
It is now essential that a mix sounds at its best on all devices, not just on a really good pair of hifi speakers.
So what you should do is find a pair of speakers that you feel comfortable mixing on. These don't have to be the best speakers, but they should be at least decent in quality.
When you have created your best mix, go and listen on some other speakers. Listen on a good pair of headphones, then listen on a $1 pair of earbuds. Then go and listen in your car. Listen in a Mercedes if you can, then listen in a 10-year old clunker.
Each time you listen in a different environment, make a note of what could be improved, then go back to your mix and make changes.
Eventually you will arrive at a mix that is the best compromise for all systems.
And after some years of experience, you will know in advance what to expect from other listening environments and you will be able to make most of your compromises in advance in the studio.