That would be really cool! Just make sure to put some kind of fencing around the area so the turtle can't escape (remember they can dig under a fence too). Also you need to watch out for predators from above. A hawk or a seagull could probably fly off with an adult RES!

And make sure you provide some shade, both over part of the pool and part of the land area, for the turtle to cool off.
As far as being happier on land, once you get your outdoor pool area setup, you'll really be able to see which he prefers. He may just want to explore a bigger area (how big is his tank?). Aquatic turtles prefer water. That's where they were designed to live and eat. They do love to climb out on a sunny spot to bask, but usually the only time they will wander across land is to find a new home (pond), to find a mate (hmmm, maybe he's looking for a girl), or to lay eggs.