LazyTanaka
Realistically, HTTYD would be the more popular of the two, especially in a world lacking in diverse quality entertainment like you explained in the novel. However balancing such a game on a time crunch while making the MANY dragon species and such from scratch would be extremely hard. At least with Real Steel he would have the ability to utilize existing templates and technology as a base and expand from there, causing him FAR less work. The game is for his Internet Cafe, so I would recommend Real Steel to gain experience and appeal to customers, then make HTTYD later for the wider market.