I find the differences to be subtle, but since the M-DAC is a tinkerer's DAC, it makes user-selectable most of the filters and other settings in and around the ESS chip so that you can probably tailor it to your taste. Some people on PFM go as far as claiming they can hear differences between firmware versions which you can up/downgrade at will... The D18 on the other hand is what it is, whatever filter is loaded and other settings are dialed in by the makers, you plug it in and either like it or not.
Not to mention that the M-DAC has a solid headphone amp section included so you can listen straight off it until you decide on your perfect amp.
I like them both in their own way, and they have found a permanent place in my two main rigs; after trying a number of DACs I'm pretty much done with that part of my audio chain despite the availability of more expensive options. I'd rather explore more amps and esp. headphones instead...
P.S.: I'm not sure what constitutes bright and harsh to your ears, but I really don't like that myself and find them both to be detailed and smooth for the most part, at least within my chain(s) (usually ending something that involves tubes). I think warmth would indicate too much coloration and rather think they're both fairly neutral in that respect... to my ears anyway.
Edited by TheGrumpyOldMan - 1/27/13 at 10:13pm