Used touring bike with fenders+rack for rainy days, new racing bike with clamp-on or large seat bag on nice days.

My rainy day commuter is a 1983 Trek 500 with fenders and a rear rack. I need the rack for my son's Burley Kazoo trailer bike anyway, so I put panniers on it.

I store extra changes of clothes at work, so on some nice days I commute on my nicer road bike without any bags; other days I use a clamp-on rack. I keep a messenger bag + U-lock at work in case I need to carry anything home or need to run an errand during the day without a rack.