@henkka, I have both and what I say is totally subjective...
If you can get both, they are so good Sound quality wise you really can't go wrong getting one over the other.
As a DAP, the M0 obviously wins. As a DAC, they feel very even but the ES100 wins out as the 2.5mm balanced SQ is better to my ears. EQ wise win goes to ES100, although once Shanling comes out with their adjustable EQ that might change. AMP wise, if 3.5mm is what is being used I think the ES100 has a more power (or seems that way) but the M0 is no slouch. . Not sure who really wins if 3.5mm is being used. Go 2.5mm then the choice is ES100.
Putting a smartphone up against a dedicated DAP is sometimes an argument no one wins. While I don't have an S8, I do have the Essential phone with the HD adapter and SQ wise I actually think my Essential phone beats the M0, however the phone's battery life takes a huge hit and lasts maybe 6 hours max. At the end of the day I use the M0 rather than the phone because battery life and usability. SQ wise the difference is so minimal that I really don't notice unless I'm using my Custom iems... And even then it's not a game changer/ender.
The ES100 is a fantastic device however sometimes I need a player not connected to anything and the M0 really shines. Especially when I'm out and carrying all my EDC gear.
Don't think you made a mistake, just save those pennies and get them both! Even if you don't/can't do that know that you win going either way!
I hope this helps and little.