You can ride both 700C and 26" wheels on trails. If you're short, have short legs or are very heavy, 26" wheels can be a better choice. They're smaller, so getting a good geometry for a short person is easier, and they're stronger. 700C is larger and has a wide selection of tires. It's used a lot for racing, so if you do group rides that may be a factor. For an extremely tall individual, a 700C bike may offer some geometry advantages.
Unless you're a serious racer or carry very heavy loads, it's pretty safe to treat the problem as "wheels are wheels are wheels".