I like Sword of the New World the most overall. At first I thought it would suck but then one day I bought it and never logged back on my SG account again. It is free now though. You really can't go wrong with either one because it is essentially the same game. Here is my old post about it.
Originally Posted by ErickDamon
I played both versions. (beta test) K2 version is not as bad as people make it seem. In fact some things are better. For example you can start out in whatever costume you like. (Actually this will be changed with the next patch and it will be the regular appearance change with armor level) It automatically goes to hold mode whenever you stop moving (you can turn this off if you want) It has more graphics options. Runs a newer version than IAH. And has way less lag for me since I live in the US.
The bad things are the voices. Changed names for characters and items. Some things are not translated or just badly translated. (That stuff seems to be fixed now) Can't customize controls for some reason. No PvP servers. (There are PVP servers now too)
So really which one you want to play will come down to 3 things. Which options you would like to have. Which version has the better business model. (both are free but cash shops are different) And which one has the better server.