I don't know either name, but I suspect this more a question of taste than anything else. I wouldn't try to find which person can "mix the best", rather who is more suited to the music you're trying to mix. Some people do really good "raw sounding" mixes, some people do the shiny modern metal thing really well, some people have their own thing that doesn't sound like anyone else, some people are hesitant to mix audible bass, some people over- or under- quantize things, etc etc etc. Go with whoever has, to your own ears, the best sounding example of something that represents what you want to get out of your mix.
But like mentioned above, it won't be as much an objective "best", but more what suits the tune(s) with maybe the mixer having their own approach to achieving that. That being said (typed) your source tones and takes better be good on their own and make sure you're prepared to have any version of mixdowns either of them might ask you for in order for them to do their job.
Also, welcome to the forum. Used a buy prestige. That is all.
Agreed with posts above, if you're well familiar with the mixing styles of both, chose the one who you would feel would suit your music best.
There's also no harm in sending both of them a message with a demo and ask for a quote from each. Their response and how much they're into your song can help determine your choice.
I understand you guys's post!!! And I have already considered it. My song is pretty both their style like happy prog stuff.
Simon should be more expensive than Adam. But in their mix, I couldn't tell a big difference. I'm only thinking Simon's mix is more open and natural but Adam is more tight and modern.
So I'm wondering who you guys like more?