Cool, I like Lv. 50, too. Basically, Open Level is lv. 100s that are three rounds ahead of Lv. 50 in terms of the level of pkmn. Level 50, yanno, starts with Mawiles and Torchics while Open Level starts with Slowkings and Wailords, stuff you probably won't see until Round 4 of Lv. 50. It depends on what you're comfortable using.
I prefer Lv. 50 because, once you know the best pkmn in each round, getting to Thorton is easy. In Round 1, Skorupi is my favourite sweeper, and Shieldon is a solid waller. Delcatty-and any other evolved pkmn-works well for damage.
In Round 2, if you can get Sharpedo or Dodrio, you're good (Stantler can't take a hit, but it's good for damage). Even better, I never have issues when I use Crawdaunt as my lead. It safely cruises you through Round 2.
For Round 3, in Lv. 50, it's a toss-up on what they give you. I've gotten Charizard and Slowking two different runs before, so Thorton was easy. Other times, I got a bunch of second stages who couldn't handle the Luxrays and whatever else he uses, so I lost. Round 3 is more of luck in Lv. 50.