It depends on the type flying you want to do. Personally I want to work for a regional next, so I took the 121 (just 2 weeks ago). I figured it would look better to have "121 ATP written test passed" on my resume as it's going to a 121 carrier; wouldn't make any sense for it to say 135. Just my opinion. Plus, the 121 is harder because there is more to study.
I cannot talk for the 135, but the 121 covers basically all of the chapters: BE1900, DC9, 737, 727 and all the 121 cross country. And of course, the 121 regs. I've heard the 121 regs are easier than the 135. On the test, I had 2 of the cross countries. A lot of regs questions, so study those well (the first half of the gleim book, basically. Just a few weather question. A lot of navigation questions.
VERY IMPORTANT: you still want to look at the 135 cross countries. I had a couple of questions on the test from them! No calcualtions though, just approach plates questions. I hadn't studied them, but they were easy so I answered them right.
All you really need to study is the gleim book, it's got everything you need. Only spend the $$ for the software if you have issues with books

I studied a few hours every day between students for a couple of weeks and took the test; I missed 2 questions only and got a 98.
If you want some hint on how to answer some of the tricky qestions, PM me and I will tell you all the tricks (very useful for the cross country planning questions).
Buckdanny