I'm partial to Wahlers, but my understanding is the two you're talking about are roughly similar in quality. All are pretty solid. As I'm no expert of German T-stats, they could all be different brands from the same parent manufacturer.
The good ones are designed to fail open, but that doesn't always guarantee that they will.
I'm partial to Wahlers, but my understanding is the two you're talking about are roughly similar in quality.
A Behr costs about a third more than does a VEMO. I've never before used a VEMO. If I buy a timing belt kit, the included thermostat is a VEMO. If I were to buy the required parts separately, I'd pick a Behr, because none of those I have bought has ever failed. I may just buy a Behr and sell the VEMO (if I decide to buy the timing belt kit in question). I'm in a similar pickle with the water pump. The one that comes with the kit is the one that used to fail, but now it's supposedly better than the other options. Go figure!
You might be paying a premium for branding. Once you bolt the outlet on, your friends won't know the difference.
But I understand the comfort factor. If you sleep better knowing it's a Behr, then spend the few extra dollars and ease your fears. Peace of mind is valuable in my book.