I have the standard, comfort and street rally seat. Its going to depend on what kind of riding you are doing. For cruising/long distance, I'd use the comfort seat. For shorter distance or more aggressive/technical, i'd use the street rally. The street rally is flatter, giving much better control but is still quite firm and if you aren't moving around much, it does start to get uncomfortable after about an hour. At £240, its also very expensive, compared to what you get in the comfort seat (Yamaha kicking back at the loophole we've used, no doubt).
I cant speak for the bagster, having not used one. Have you considered something like an Airhawk?