I'm going to recommend with some firmness that people don't switch from mGBA to the RetroArch port of same, "nice UI" or not. For one thing, RetroArch mGBA is probably lacking any number of features that the natively built version has: e.g. games with motion control or which use the solar sensor probably aren't playable at all. Also, keep in mind that mGBA is not exclusively a piece of software developed for 3DS. It's been a PC emulator for much longer than it's run on 3DS, where it is being tailored to the 3DS hardware. The RetroArch port is not based on the 3DS branch specifically, it's just a port of the standard core to the libretro architecture and will likely fall behind on 3DS support as mGBA 3DS becomes more specialized. You can already experience how performance of RetroArch mGBA lags behind the 3DS version with games like Kirby: Nightmare in Dreamland, where RetroArch runs the game at about 2/3 the speed native mGBA does.
What RetroArch does is many times great. What it's probably not great for is porting emulators to platforms they've already been ported to by their original developers with care taken to take advantage of the new platform. RetroArch is fundamentally about making things the same across many platforms, so cases where its core will outperform the same software built specifically for a platform are likely exceedingly rare.
Sorry if this sounds critical of either the goals or implementation of RetroArch, which is not my intention. I'm simply making the case the goals of a jack-of-all-trades project like RetroArch are very different to software that's tailored to any additional platforms to which it's ported. The RetroArch approach gives us great things like relatively quick ports of emulators to 3DS once they're plugged into libretro as a whole, while mainline mGBA's approach basically means less platforms run mGBA but when they do it's done with consideration for the capabilities of the machine running the emulator. Phrased another way, on platforms where mGBA is not currently available, the possibility of running the emulator via RetroArch is a massive improvement over the alternative, even if that means it's not going to be tailored to your hardware. The thing is that mGBA is available on 3DS.