I like tri-shorts for rides 200k and under, but eventually I think I want a little padding. I'll have to try a 300k that isn't an important one in tri-shorts some time. It hasn't bothered me (at least not to this extent) before.
Anyone have any recommendations for shorts that have more than just fleece but breathe? (I'll look into the Castellis, although IIRC their XL is a little too snug on me. (Terry XL fits me great.) I've also had leather saddles recommended, but I really try to avoid leather. (Though I might be playing saddle roulette anyway; the underside edge of my saddle was annoying me as well, although that might have been because I was sitting differently to keep pressure on my bladder down.)