When one of my wholesale customers (mechanic or shop owner) wants an O2 sensor, they get the one with the factory plug on it. Even the cheapskate ones still don't buy the universal ones, so take that for what it's worth. Even if a sensor reads bad because of a faulty wire, most of the time they replace the sensor rather than repair the wire.
As far as the best way to make connections, I've always been taught that a proper crimped connection is superior to soldering. This assumes using the proper size connector for the wire, using a quality connector, and using the proper tool to make the crimp. Soldering is harder to do properly (for most people), less flexible and less resistant to vibration. I'm not saying a proper solder joint wouldn't work, and the crimp vs. solder debate can go on forever.
In the case of external wiring on a vehicle, I would use crimp connectors with heatshrink over them to protect from the elements. A properly done crimp will have no appreciable voltage drop on a 12v automotive electrical system.
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