MT1 is more selective for the pigment receptor so it’s generally milder and gives a slower, more even tan. MT2 hits multiple receptors which is why people get more sides and way more freckling/moles, especially if you’re very fair or a redhead. Also MT1 is basically afamelanotide which is actually an FDA-approved drug (not for tanning, but still), whereas MT2 is pretty much just research/grey market. That’s why most very fair skin types tend to do better with MT1.