I buy a little of both, I've bought like 12 booster boxes since 2013 from various sets and haven't pulled a single secret rare from any of them. However I have pulled like 6 secret rares from singles/promo packs I bought at Target or Walmart in the last month.
I usually have really good luck with promo boxes and single packs. An example would be in November I bought like 10 Furious Fists booster packs and 4 Phantom Forces packs at Meijer, I got Mega Lucario Secret Rare, Lucario Full Art, Gengar Full Art and Dragonite Ex, then the next day I went to another Meijer bought like 9 Phantom Forces packs and got Full Metal Dialga, Full Art Manectric and Mega Gengar Ex.
But booster boxes are a different story, my first BC box had a Full Art Celebi, Full Art Bianca and 3 ex's, which is great I got new cards. The next month I bought another one and got 1 full art, Celebi...I've pulled 3 Full Art Celebi, 2 Full Art Cresselia, but I have yet to see Full Art's of the other 4 Pokemon.
Today I bought the Mega Diancie Box, it wasn't as good as my Mega Metagross boxes but it did have a Fossil Researcher Full Art and a Team Flare card.
I also got 3 tins, here's the best card I got.
This card finishes my Plasma Blast set excluding secret rares.
Only 5 of these are from booster boxes.
Here's the whole picture
In case you're wondering the Reverse Pokemon Catcher and the Genesect Full Art were in the same pack.