Whether or not to stay in Springdale or St George really depends on what your itinerary is.......

Its about a 45 min drive from St George to Zion National Park so there would be a drive to get to Zion if you stayed in St George. Usually for a visit just to Zion National Park, its suggested to stay in Springdale since its located right next to Zion National Park, and is more convenient for visiting that area. Heres are a few threads on the subject.

http://www.tripadvisor.com/ShowTopic-...

http://www.tripadvisor.com/ShowTopic-...

However, some people will choose to stay in St George for a variety of reasons including the interesting things to do in the area in and around St George, and because the lodging in Springdale often may not have availability for thier stay.

Heres a good thread thats lists things to do in the St George area to help with making a decision on which area to stay in.http://www.tripadvisor.com/ShowTopic-...