I've owned and flown behind both as PFDs in my RV-7. I had a fully optioned Mini and loved the significant functionality offered. It really is a complete EFIS in a small form factor. I took a CRJ first officer for some acro and he commented on the flight path indicator and how he needed to step up to jets to get one. I smiled.
Pros/Cons - upgrade path, more complete feature set (than a G5), slower startup to an attitude indication from a cold start / reset. The square hole mounting increased my installation time as I was starting with a 3 1/8 and a six pack. I didn't end up installing my second Mini (wanted a PFD/HSI/MFD combo) before changing panel direction.
I decided to go with a dual G3X and switched to an early G5 as my PFD (which would later become my backup). The G5 powers on quickly and has an attitude solution immediately (while it's still aligning). The display frame rate is super smooth and the number 'tapes' scroll nicely allowing trends to be easily identified. I like the fact that Garmin produces a 'substantially similar' certified unit. I miss functionality such as; OAT, AOA, wind vector, true airspeed, density altitude, ADS-B traffic, synthetic vision and so on that the only slightly larger Mini has. Yes, the G5 has a smaller display and I concede that it would difficult to display all of that information. I do feel that a OAT, DA, wind vector and perhaps a simplified AOA display could be integrated. I've flown the G5 for a few hundred hours and have found it to be rock solid. The pace of updates is slightly more frequent from Garmin. I've found GRTs support to be excellent and I like being able to speak with the engineer who decides whether my feature request will make the product development roadmap.
Dual Mini's would provide more functionality including mapping and engine pages. Dual G5s with a 660 would provide similar functionality (minus the engine display). I like that there's choice in the marketplace. The G5 offers really nice integration and backup for the G3X and the GRT Mini offers access to a larger (including legacy) ecosystem of navigators.