I have an Opus, so you can maybe take what I have with a grain of salt. I also have an IPV4 that I love. The IPV has a Yihi chip as well, although I certainly understand it is no SX350J.
Given the price of the SX Mini and the Opus, I would say go with the Opus. It really is not that much bigger, the battery life is likely similar because the M Class is on a single 18650, and the Opus has all the power and customization/tuning options you could possibly hope for. I was thinking about getting an M Class until I saw that there was a DNA200 mod out, which was the Opus, and I couldn't be happier with it. If I bought the M class, I know I would have regretted it as soon as I started seeing everything that the DNA200 could do (beyond just the power difference), although that's just me (I get bored with things quickly and always want the newest/most advanced).
Although we'll start seeing rather soon, I imagine the only difference between the Opus and any upcoming DNA200 device will likely be aesthetic/size or in the battery, and was really, really surprised and happy with both the size, looks, quality, and battery life (although it could be slightly better). That all said, I know there are a lot of people equally happy with the SX Mini and don't want or need the extra power or features that the DNA200 offers.'
Edit: Oh, and the Opus is from an American manufacturer with a six month warranty (something others probably could not care less about, but both of those things are fairly important to me). When I ordered my Opus, they said the next batch should be out soon and that was over a week ago.