I have heard both of those amps in the same system. Michi delivers a lot for the money. I think some people who have never heard a Michi amp will "knowingly" relegate Michi to a lower status because it is a Rotel product. I'm always amazed at how much some folks know about a product without any actual experience.
I have auditioned Michi against McIntosh and Classe products and much preferred the Michi-obviously a personal preference. The Michi could power a small city, is built like a tank and seems to be well engineered. I find it to be a bit warmer than the Boulder but nowhere near the McIntosh sound. I think its a bargain and a very good performer even if you don't consider price.
As much as I like the Michi, I don't think it is quite in the same league as the Boulder, which I think is an elite brand. The Boulder amp is about as close to dead neutral as you will find-it neither adds nor subtracts. Some might call this analytical, not sure I would. Very clean sound and a superb product. If you are looking to add warmth, not for you. If you are looking for neutral Boulder is an excellent choice and its "entry level" range gives you much of what you get from the superb but incredibly expensive higher ranges.
Just my thoughts, everyone hears differently.