MadMonkey & Fencer: Maybe I should answer that question, because I was the one requesting PRO go-moku in the first place.
Between the REGULAR go-moku and this site`s 5-in-line, the only difference is that in go-moku overline (more than 5 in-a-row) doesn`t win. That difference has not much effect to the game.
However, if the game in development is PRO gomoku the difference to 5-in-line is significant. In pro gomoku there are 2 restrictions to the white (starter):
1. The first move must be in the center of the board.
2. (much more significant than 1.) White`s second move must be outside the 5x5 square around the 1. move.

The 2nd move restriction takes away some of the white`s advantage and makes the game quite different.
The most important thing being that in pro gomoku there is not a sure win situation for the white in the beginning of the game, as is the case in regular gomoku (or 5-in-line) on a 15x15 board.
Not all but quite many of the advanced players prefer the pro version (some are actually waiting for it before they start playing here): it is an even more complex game. Important thing is also that in pro almost every game is unique, which is in contrast to regular version where the same moves are played over and over.