Behr vs Wahler
 

Which is a better thermostat? My engine's running cool, and the tstat looks pretty old, so I'm wondering if it's stuck open. Should this throw any codes?

Been doing BMW's since the early 80's, BMW has used Behr & Wahler as thermostat OEM's - depends on the car / engine. Either would be brands I have and would use.

If your car takes a rather long time to warm up (gauge, not to make heat inside), it could be the thermostat. Cheap & easy enough to replace, even if it's NOT the case - you haven't spent signficant money and you've proactively replaced something that is of value.

Codes would only be if the late warm up causes an issue with the emissions control or similar.

If the car takes a VERY long time to start making heat - this could indicate a potential issue with the electric heater valve.

Hmm... heat seems to work. No codes, and otherwise running smooth, with the exception of some belt noise at startup in cold weather. Going to do the belts and tensioners in the spring. Pulleys are tight.

Open Tstat usually throw code P0128 (engine temp under threshold), but not always. My old 3.0 had the open Tstat without code. I knew it's open since the temp needle never get to the 12 o'clock position, more like 11. Whenever I'm running on the hway, the needle will go down to 10 o'clock position. Heat will take a little while to come on.
Between Behr and Wahler, I would choose Wahler. Behr had parts (not all) made in China now, and we all know quality control over there is not the BEST in the world ( :bustingup )
While you are replacing the serpentine belts, give the pulleys a spin. There should be resistance in them. If they spin freely, replace them.

Thanks, guys! Will double check pulleys when I do the belts, and will probably swap out the water pump then, too jic. Now to find the pt numbers for the hoses.

Which is a better thermostat? IMHO that would be what the MFG recommends.;)