Which Would You Choose?

Neither. I would take the JC 5 over either one. Not sure why you think either of these would be an improvement. 

I auditioned the S5 against a Luxman M10x. The Luxman was clearly better, as it should be for $20K. However, the Michi was very impressive for its price. It's definitely a great value in this crazy, high-end world. However, depending on your listening preferences, don't rule out a McIntosh MC462. The S5 is slightly more detailed, but the McIntosh presents as warmer and more real with a tremendous bottom end. 

What specific improvements are you looking for over the JC5?  Also, I’d consider upgrading the Monster 3600 that can’t be doing you any favors sonically.

@soix 

+1