Since both sites deal primarily with weather, I assumed they would have similar offerings. As it turns out, each has a unique approach to providing travelers with weather information.
Whereas Weather Underground provided a detailed snapshot of the type of weather you could expect in a destination for particular dates (even down to how windy it might be), Weather.com's Travel Smart category offers a pared down average of highs and lows beyond the reach of the 10-day forecast. However, Weather.com has a number of other helpful tools, such as information about the weather at your departing and arriving airports and what-to-pack suggestions (though it might be too general to be of too much use).
There are also other weather, traffic, and travel-time forecasts, but I think the coolest thing in its Travel Smart category is the Scenic Drive Ideas, which offer (surprise!) road trip ideas in a number of states. Along with the Drive Ideas is a tool that provides information on the weather along a particular driving route, and as someone who once drove straight into an unexpected (to me) storm in Arizona at 10:00 at night, with rain was so heavy I had to pull off and wait for the worst to pass, I see the value in route-specific weather forecasts.