When looking for a race, people usually are looking for flat or downhill courses. Two years ago, when volunteering in Richmond to present the Pittsburgh marathon I was surprised to see how many runners were reluctant to sign up because of the elevation profile. The city is known for its hills and the race directors did a great job to avoid steep hills (look at the five steepest hills to get a general idea). But when presenting the course, you have to come with an elevation chart to convince them it was not too hilly.
The general idea is that a flat course (or a downhill one) will be easier on your legs and require less efforts. Of course, this is obvious you are going faster on a flat course but was the elevation never change, you are working the same muscle group. And if you choose a downhill race, this can also put a lot of stress on your joints. Even for a flat race, you are putting all the efforts on the same muscles group and exhaust them until the last mile. If you are going to try to qualify for a particular time (Boston qualifier anyone?), this can be (1) very exhausting and (2) hard to recover as your body is not used to such long and intense efforts on the same muscles.
That might seem odd but running a race with some elevation variation can be easier on your body. As your running form is changing according to the elevation, you are not using the same muscles and when a muscle group is used, the other can rest for a while. Of course, this will not be easy, but if you are trained and used to take some hills, it will be definitively easier.
One example on the east coast is Erie and Pittsburgh. Erie is a big Boston Qualifier race: very flat, the course takes place in what one might consider as ideal conditions (perfect temperature most of the time, completely flat race). This is a two 13.1 loops on Presque Isle. Needless to say, pretty boring: after the first loop, you have only one wish, finish as soon as possible. On the other hand, Pittsburgh (or even Richmond) have some slight hills and is considered more difficult. But as long as you are used to run some hills, your body will recover quicker. And the course is nicer: you experience most of the neighborhood and the volunteers are pretty awesome. No matter if you are a Pittsburgh native or a new visitor: you will experience the city from a different perspective. Better than doing two loops on the same course. If you never experience the Steel City, I just recommend to come and experience this race. This would show you why Pittsburgh is one of the best city to live in the USA.