I've gone through this over the years and come down to these options:
1. The easiest most versatile option has been zip off pants. Ex Officio and REI are my favorites.
2. Running shorts and wind pants. This is the lightest combo that I've found. Most running shorts have a light (and non-chafing) brief built in too. In fact, good running shorts don't weigh much more than a lot of briefs. The caveat is the durability of the wind pants. I have a have a pair of Montane's that are certainly ultralight, but they need a lot of caution to stay in one piece.
3. If weather is going to be more variable, then running shorts and rain pants make a good combo. I think it is a waste of space and weight resources to carry both wind pants and rain pants. For Summer hiking in my climate, if it is cold enough to warrant long pants, then it is raining too. Even in the height of Summer, hiking early in the morning with wet brush can get me as wet as any rain. Wearing rain pants until the dew evaporates saves a cold soaking.
4. Operating in cold wet climates like the Pacific Northwest Fall/Winter/Spring, I find it easier to forget "normal" pants and use rain pants with varying weights of polyester long johns. I like Marmot Precips with anything from silk weight Capilene to Power Stretch bottoms. You get all the wicking properties of the polyester and they are great to sleep in. The outer layer is going to get wet with brush and muddy too, so you wouldn't want to wear them in your shelter anyway. Full zip pants weigh a bit more, but allow good venting options.
Short gaiters help with any long pants, keeping them out of the mud and rocks or catching the cuff as well as keeping the crud off your laces.