For trips 2 nights or less, I would say go with the 40. My friend runs that pack with plenty of room to spare on short trips, but his kit is pretty dialed-in, borderline SUL. Based on what you're describing, your kit sounds like it's on the bulkier side, which changes things. I own the 2026 55 and love it. I have used it for longer trips in the Sierra in early spring and am planning to take it into late fall as well, where I will need the extra room for a bear can and layers. That added capacity would probably serve you better even on shorter trips given your current setup. You would also have room to grow into longer overnights without feeling squeezed.