The last thing I want is my bung sitting in water or condensation.....
But yes, it's internal condensation. You're already introducing a condensation point, for lack of a better term, with any irregularity in a smooth tube; making sure water doesn't pool when the car is not running and corroding the sensor would seem to me to be the most important goal. It's all going to burn off once the car warms up, but a low spot with external welds means you have pockets for water to sit in and rust/corrode out.
I'm not going to pretend to be an authority on placement, but the DP/midpipe, depending on packaging, is going to be your best bet. The usable life of the O2 sensor is going to be limited by contaminants more than heat in that spot anyways. Even a tailpipe mount (which isn't claimed to be superior, just a much better option than no wideband at all) that doesn't require permanent mods, and is all the way at the end of the exhaust stream, still gets gunked up and wears out after XXX miles.
It comes down to the optimal location, and then the location that gives you more than acceptable results when incorporated with other logging and gauge readings. Unless you're tuning a car to the bleeding edge, using a tailpipe clamp is going to let you keep an eye on AFR's in concert with all of the other information you're gathering to monitor the engine for a good, solid, daily-driven tune.