One of the longest threads on the BobVila Fix-It For'em has to do with this topic. There are lots of opinions posted here, and if you look at Jay-J Moderator's response near the top of the thread, some useful links. http://www.bobvila.com/wwwboard/messages/42356.html
I have no experience with the siding and cannot make a recommendation; however it appears that contractor experience, reliability and service are extremely important. Be sure to request references, and check how long the contractor has been in business. Understand warranty limitations.