Your local code determines the material of the pipe. Here in the city of Philadelphia, Plastic pipe is not allowed to be buried underground by code. However they let you use the fresh air inlet as PVC, So the code is contradicting. Check with the plumbing inspector, then go from there. Roots are always are hassle no matter what pipe you use. You may have to get rid of some large trees that may cause problems down the line. Think ahead here. Cast-iron pipe is a preferred materail for underground use, but then we have the root problem to consider. Good Luck, Tom

Depending on soil conditions and local codes I would use Cast Iron