Either Gates to Infinity or Explorers of Sky.
If great presentation is important to you, get Gates to Infinity
If the quantity of content is more important to you, get Explorers of Sky
As for the story, I'd say Gates to Infinity had the better one by quite a long way (The Wigglytuff Guild was useless) but both games have stories that are drastically above average by pokemon standards and are to date the only good stories the franchise have ever told as far as I know
