I prefer high quality paint to undercoating. If you get a scrape or scratch in the undercoating, the undercoating will hide some developing rust until a big chunk of undercoating falls off. I never had any spectacular results with undercoating but I did use a lot of 3M. I did the underside of my truck in moisture cured urethane 2 years ago. It's still too soon to tell how good it is but it is definitely holding up better than the epoxy primer and polyurethane paint on the body. I am going to do the body over again in MCU this year. This is the stuff if you're interested.
http://www.masterseriescoatings.com/index/