Which Would You Choose?

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. 

It seems that if in fairness to those seeking an opinion, those rendering an opinion should give the basis for the opinion. If you believe a component is "midfield" whatever that is, you are certainly entitled to your opinion. Obviously, if you have no significant experience, actually listening to that component, that affects the credibility of the opinion. I don't have the ability to judge equipment that I've never heard but perhaps others do