I had the Q3 Emille briefly for a day or so (did not use it in the field) but returned it and replaced it with the Arca Swiss Z1. I think the closest competitor to the Z1 would be the Markins version above the Q3 (M10), even though the Z1 has more load capacity (130lbs for the Z1 vs 88 lbs for the Markins M10). The Z1 probably has more load capacity than even the M20, I believe.
I think the Q3 is light and small and very well built, based on my brief impressions but the pricing was too close to the Z1, which is what led me to go with the Z1.