I grew up eating Khao Soi made by my mom since we are originally from Chiang Mai. I've tried Khao Soi at Sticky Rice and I would have to say it is sub-par. I'm surprised that IMMM has the dish too but so far I'm not too impressed with their curries (too sweet and too much coconut milk). I haven't tried Khao Soi at Andy's Thai Kitchen but I would rather place my bet on Krung Thep since the chef there is from the northern parts of Thailand and other northern Thai dishes on the menu were fine.
Khao Soi is in fact simple to make and I usually make them fresh at home since most of the restaurants make them in large batches, freeze them by portions, and reheat them once ordered. You can find all the ingredients required to make the dish at the Vietnamese markets in Argyle. Best of luck on finding the right one for your taste. I'm glad that Khao Soi has been gaining international attentions. However, as a Chiang-Mai born person, recreating the taste at home is the only way I can satisfy that craving.