I run both a monthly campaign and a weekly one. From what I've seen, it's the monthly group that is much more cautious and the weekly one that acts braver, but usually not to the point of doing stupid actions as you described. Some of the variation is likely a result of the party composition and alignment. The monthly group is very lawful good to neutral good in orientation, while the weekly one is a bit more chaotic.
Monthly - LG Fighter/Cleric*, LG Paladin**, NG Bard, NG Sorceress, NG Ranger, LG Monk
Weekly - LN Monk*, NG Cleric**, NG Bard, N Fighter/Barbarian, N Druid, CN Rogue
*, ** - Indicate characters run by the 2 players that participate in both campaigns. The other players are in just one or the other.
Thus, just in roleplaying their characters, I'd fully expect the weekly group to be more prone to risktaking, which is exactly what I see.
The biggest problem with my monthly group is that we have a very hard time even meeting that often. In summer it's lucky for us to play even 1 time in 3 months with all the busy schedules, but we try to play a bit more often in fall, winter & spring to make up for that. It's tough to remember all the details when it goes much more than a month between sessions.