On the good better best line of thinking 2" blue board is the best, it's what we're using in my professional life under all slabs heated or not. New energy codes up here are super strict and we are using 2" blue dow under slab to help meet insulation package codes.
BUT:
I have 1/2" bubble insulation with the white face on one side and the foil face on the other, I placed it with the foil face up. Keep in mind I built in 2008 and that was right at the time that it was being realized that the insulation under the slab should be a foam insulation not a bubble with foil. That being said, my garage and basement heat fine, I did it like we had been doing under-slab stuff in the early 2000's and late 90's. Is it the best nope, does it work, YUP!
Take a practical look at it, like I said my opinion on floor heat is that it is great for laying under a truck and working, but sucks for changing the temp of the air space in the short term.
One quick story about in floor heat: I have a client that bought a house with more issues than Time magazine. The property is simply awesome but his new to him 13,000 SF house had heating and cooling issues big time. We found heat that was locked on etc. So in testing each zone of the house we manually turned on one zone at a time in the house and let them run wild for 48 hours, then walked with a infrared gun and mapped where each zone heated. When we got all done we found one area that seemed like it had no heat in the floor by looking at the map but when walking the rooms it felt fine. We kicked on the mystery zone for like a full week and found that it heated up and connected all the dots and we had a full house map showing each zone and that we had 100% coverage throughout the house. But then the room the mystery zone was in wouldn't cool back off even with the heat lines shut off manually. We thought there was some sort of cross over for a while, but we soon discovered that room had been an old garage or something and the floor was like 18" thick with heat in it. It takes forever to heat up hence the reason we couldn't figure out where the edges of the zone were with temp guns, but once we heated that 18" slab of concrete up for a week it took it like a month to cool back off since it stored so many BTU's.
My theory is even if you are heating the ground a bit it will warm up and become part of the thermal mass. Again is it perfect, NO. but it does work.
Edit: Having the floor mass warm will aid greatly in retaining heat when you open the doors to move stuff in and out. The air might get cold quick but that big slab of warm concrete has a lot of BTU's stored to give up.
