I have had the 50 amp one for 8 years and have been very pleased with it. I would recommend the the 50 amp vs the 30 even if the rig you have today is 30 amp because you might by a new rig with 50 amp. I have provided a link to an installation article that discusses the surge factor. I don't use one.
http://www.geocities.com/genieyorks/autoformer