Brands? Madbull, Guarder, Systema, Prometheus
I've used all with success. Sometimes you have to test different ones, though.
My UMG does great with Madbull, my P90 hates Madbull but loves Guarder, my ICS likes Systema and Madbull but not Guarder. I know, it's wierd...
As for hard vs. soft, hard buckings are for higher FPS setups (350+), soft buckings are for sub-350 FPS. This is a general rule of thumb, though. Some have other theories, but this appears to be fairly common accepted school of thought. I run hard and 60-70% buckings in most all my guns.