0W-20 and 5W-20 must meet the same viscosity at room temperature (which is basically the same as the outdoor highs you cite) and at the elevated temperatures the oil actually will reach in the engine.
OW, 5W, Ten Thousand-W (if it existed) all only refer to cold temperature, basically below 0F, performance only. Using the Winter viscosity rating (xxW-20) tells you nothing about the room and elevated temperature viscosity.
The 0W-20 could easily have a higher viscosity than the 5W-20 at engine temperatures, since the SAE 20 spec is a range, not a fixed value.
However it's a narrow range, and both oils clearly meet SAE 20 since the rating is not optional ... you can't call an oil SAE 20 if it tests out at SAE 30. So they both will be similar viscosity. The only significant difference will probably be price.