I always use Fast Poster first. Slow Poster takes to long, but if you have the time.
I use fast poster in SB too. I honestly don't know the difference between the two, especially if you're using 25+ private proxies that are automatically rotated in SB. When I do the fast poster, it's always using 40 connections or so. I think it's just mostly has to do with the CPU/RAM/Internet connection so SB doesn't rape your little eMachines christmas prezent.