I haven't used mRing, but plan on it once I get a free moment. This summer has been very busy -- I've been travelling quite a bit for weddings, communions, etc.
Anyway, I liked Ringo Pro, but had some issues with soft resets during incoming SMS messages. It didn't happen all the time. However, when it resets, you're phone is off. So, I'm sure I missed a few calls. For the most part, it was stable. I'm going to delete it and try mRing. For me, stability is key. If mRing delivers in that area, I'd probably side with mRing. Many people have great success with Ringo Pro. Either way, I'd suggest running the trial first. I really liked the Ringo interface - simple and the functionality was great. I did a review of
Ringo Pro if you'd like more info on my experiences with it.
-Chris