CharlieZulu -
You are correct, the FAA still takes the writtens away at the checkride, and your license does not reflect which test you did.
Minus 273 -
I agree with you on the fact that the 135 test the regs are a lot similar to Part 91. Hence easier to work out, plus you only have to deal with the Beach 1900.
Pete -
If you look in the front few pages of the Gleim book, it explains which chapters to leave out if you are doing it Part 135 or 121.
The only reason the FAA has 2 slightly differnt papers as most people in the US only do there ATPL after working for an operator for a while. As you do not need to be an ATPL to be a First Officer, even in a 737. Thus, if a pilot getting ready to upgrade in a 121 operation would prefer the 121 test and similarly a 135 pilot.
Regards
Speed