To answer the question you posed, I own the Michi S5 and I have been very satisfied with the performance. As someone said, it's probably not going to compete with an amp three times the price, but that should be no surprise. I recently was looking to upgrade, and auditioned against Macintosh amps and preferred the Michi over the Macs and Classe. I also listened to Boulder as well, which certainly was better but of course at about three times the price. In my case, i thought I could do better by keeping the michi and spending the available money elsewhere. I find the Michi gives you abundant power, and pairs well with SF. So, I think the S5 offers a lot of performance for the money.
I have not heard the MoFi. I don't have the ability to review components, I have not heard, at least with any hope of offering a helpful opinion. As with most buying decisions, I think it comes down to getting the best performance for the money you have to spend. Obviously, if money is no object, upgrade everything.