funner = 380,000 google hits

more fun = 18,600,000 google hits.... more fun wins by a margin.

(I would say "funner" when I want to make a joke about not knowing how to speaka da inglish.)

However-

"The editors of Merriam Webster�s 10th edition choose to recognize the word �funner� as a lesser used inflected form of the adjective fun. They do this because they choose to regard English pragmatically, taking into account colloquial and dialectical usage when they make their editions.

I would make the argument to a student that the -er, -est construction on an adjective is permissible. With no clear abstract rule to determine which adjectives are to take the �more + adjective� or the -er, -est construction, the �rule� becomes arbitrary. The �rule� I would argue falls more into the area of convention and accepted usage than grammar.

The person choosing to use this construction should be careful when and where they use such a colloquial construction as those familiar with the �rule� may take it as a sign of ignorance.

See http://www.m-w.com/dictionary/funner"

http://www.answerbag.com/q_view/152567