Check your local jurisdiction/municipality, I have worked in different jurisdictions where they preferred one over the other. As stated earlier it really comes down to maintenance.
less than 2' typically requires CL.IV pipe. Also word from the wise, the class of pipe and type of bedding is extremely important the deeper you go. Over 14' of cover (9' to be conservative) check ACI and ACAP (http://www.concrete-pipe.org/index.htm)
For elliptical pipe refer to the following...
