Essentially it's just what I got used to in that year of riding only the G15.
When I put the MT7's on the G13, the first few rides I had some inconsistent brake line pressure in the rear, which was later resolved through a better bleed, and then the rear pads wore out and had a few little quirks replacing the pads and re-bedding etc. My initial feeling of going back to the MT7's was just noting the different power ramp and bite points, which had me blowing out corners and lacking a bit of control. The G-Spec's showed up on a quick Facebook sale around then, and I just jumped on it for the opportunity to normalise the quiver and address "re-learning" my braking every time I switched bikes.
That being said, after the line pressure and brake pads were resolved, the MT7's are just an amazing brake, and that partially contributed to me being slow on switching them all around. I feel if I had put the MT7's on the G15 when I had initially built it up, I'd be in the position of grabbing another pair of them, or MT5's for the G13 instead.
The Quadiems fit in my criteria of a four pot brake, mineral oil based, but had the advantage of being on sale at the time I bought them (think it was a 1st July Sale in the states, with 50% off) and a silver finish; so were aesthetics driven, and also just a "lets see how they go". They have been completely fit and forget, even with the switching them over between bikes and cutting lines, they just went in and didn't even need bleeding. I burped the lines the typical Shimano way of just jamming the bottle in the lever port and pumping the brakes a bit, but it wasn't required for anything other than peace of mind. They're simple, and they work. They have an amazingly sturdy lever feel; the Magura's don't quite have that sturdyness in the hand, lever flex was noticeable on the longer lever, but they don't feel as "locked" into the lever, there's some slight up and down movement on the pivot point. But the MT7's are a fair whack lighter as well...and the magnetic brake pad installation is inspired. Quadiem's can take Shimano Saint brake pads though, and are just robust and reliable.
I honestly couldn't call it between them, my primary motivation was I got a good deal on them and they're consistent in feel between pairs. Both bleed methods are near identical (and Shimano still holds the crown for being easiest and most straight forward with next to no "tricks" required). I will say the Magura needs at least the HC lever upgrade, if not the HC3, so factor that into the purchase. If you count grams, the Magura's will win out, otherwise flip a coin an enjoy.
HOLY SHIT that ended up being longer than expected. I need to go for a ride but it's blowing a gale here and cold here today, so this was my procrastinating.