I wouldn't leave either in the car. Extreme heat isn't good for ANY CALL and can ruin them. I carry my calls with me every day. I stick them in the backpack that I carry my computer around in. If you have to leave it in the car, make sure and keep it out of the direct sunlight.
As for the selection between the SB and the Timbre, I can come close to giving you a comparison. I own the Daisy Cutter and the Timbre, both in acrylic. The DC is a variation of the SB. Now I'm definately not an expert, but I can tell you the difference between the two. The SB is going to be raspier and have MUCH more volume. You can still be quiet with it, but you CANNOT be very loud with the Timbre. The volume produced by the Timbre is MOST DEFINATELY loud enough to call ducks. If you want to call them from a mile away, don't get the Timbre. The Timbre has more of a nasal sound and was designed for close in work.
I'd have to say that the Timbre is my favorite for hunting, but I have more fun blowing the Diasy Cutter (high balls/hail calls).
Here's a thought, buy the SB custom call and go get yourself the new Quackhead Timber for $25. It is a poly version of the Timbre call. Christian, the co-designer of the Timbre, claims that the poly version will be the only call on his lanyard this season.
Listen to the calling on this page, one of those calls is the Quackhead (RNT) poly Timber.
"I'll start spending less time with my dog and more with my wife when she starts fetching ducks for me"