AC flanges are 6 in. I bought the daystars because the light pattern they put out was more concentrated and less diffuse than the radiants. Since my rooms have plenty of coverage I thought that was the way to go, versus going for more coverage at the expense of output at the canopy. As I recall they were about $20 cheaper too, although that wasn't a huge factor. It was all guesswork tho, as I still have no clue what I'm doing.