It is my opinion that GenCon is really not focused on Miniatures wargaming. Origins is where you'd find the most people who would sign up for a game, just because it sounded cool. Gencon would be good to get some exposure to a "wider" demographic, but convincing a 12 year old kid who plays primarily Yu Gi Oh and Magic to play a miniatures game is going to be a hard sell. Convincing a Civil War/WW2/Napoleonics Mini's player to branch into Sci-Fi, much easier.
Being at both, in an optimal world with unlimited cash resources would be best, but I suspect Origins would be easier to fill events at.