Noticed you'd had no reply so thought I'd chip in
Not much info around on the MFC's, but the ones I've seen are best described as budget fins. I think Micah B used to use the (so they can't be bad) but most seemed to opt for other brands.
Sonntag are very good, but they do a few different models and you need to make sure you match the right one for conditions you want to use it in. You could contact Jorg at Sonntag for advice, he is very good and knows his stuff. Here's a link with some useful feedback on these fins (if you speak French):
Or, you could look at other brands that are known to work with iSonics. Z-fins (big money and long-ish waiting list). Select V-max 41cm and 43cm. F-Hot (my favourit) 40cm and 43cm. If I was just getting "one" fin in either Deb or F-hot, I'd be going with a 40cm as a decent all-rounder.
The problem with your two current fins are that the Select S12 is a powerful light wind shape and the Deb SL3 is a high wind shape for extra control. There is only 3cm between them but their designed range off use is for opposite ends of the spectrum.
Just my 2 cents worth