121 v. 135 ATP
It makes absolutely no difference which ATP written you take. Don't think of it so much in terms of difficulty; think of it in terms of the one that's easier for you to prepare for.
For me, going to class was easier than trying to study at home. I went to a weekend school. The class was based on the 121 exam and featured B727 performance. I knew nothing about turbine equipment or 121 or 135 regs. I was the only lightplane pilot in the class. I had an excellent instructor and I studied for ten hours the next day. The day after that, I took the exam and scored in the 90s.