I always hated the treadmill and would much rather be outside, even if the wx was crappy. Rain, snow, freezing rain, heat, cold, whatever, as long as the pavement wasn't icy and the windchills were above zero. Some of my favorite runs were early, oh dark thirty, with a light mist or even fog. Peaceful.
Running outside lets you set your own pace. Going uphill or just need to dial it back? Go ahead. Feel like pushing it? Do it.
The treadmill maintains a steady pace. Sure, you can change it if you want, but it isn't as simple as just speeding up or slowing down.
Later, after I wasn't so competitive with myself, I ended up preferring the treadmill. I could do it in the climate controlled comfort of my own house. Add a fan when necessary.