Sohc engines are old, brittle, testy engines...if you keep it cool, and don't overrevv it, it will be kind to you. If you put a metal headgasket in a SOHC (you gotta look hard for one...they're out there though) it will last a while. DOHC engines are just better, newer engines...more power, more tq, more tuning capabilities, a much better aftermarket, and very reliable from what i've heard. They still make Dual Cam KA's (14 years with minor changes in the ignition system and other things) in the 4cyl frontiers....so technically you COULD swap in a dohc with 5k miles on it if you wanted a truely reliable engine... It all comes down to money.