I love these theoretical arguments about better value or which bike is superior, it is futile. For each person the needs/tastes are different and really the only problem is that in the usa one can't just go and test out a minivelo to see which one fits you better or just appeals to you more for whatever reason. I bought a mercier nano a few years back and I liked it but it didn't really suit me and my riding style and I ended up stumbling onto my origin8 bully at a local bike shop and was able to test it out and it happen to be the answer to my mini velo dreams.
whichever bike you choose you can most likely make it work for you if you are so inclined, for the mercier yes you have to search far and wide for a good headset and if you want any kind of performance you have to replace almost every part on the bike but that can be a long process spreading the expense over time making it a more economic choice in the short term. the soma is a very nice little bike but it is in limited supply and on most of those listing on ebay the $399 is a starting bid not a buy it now, but if one were so inclined to get fat tires for that bike you could put 406 rims on it and with 1.95 tires which would have the same outside diameter of 451 wheels with 1-1/8" tires although I am not sure you could keep your canti brakes.