I personally am in the "2" bike shorts camp, just cuz I think its better being in clean ones, and how they can often not be dry overnight. I figure the weight and space is really nothing, shirts and such generally dry quicker and I can live wearing a dampish shirt, as it wont be dampish for long.
plus I have always used the "bungying underwear or bike shorts" to the top of my panniers as effective anti-theft devices as well as drying them.