DEEC, by what you say I understand that with the ATP121 I would not be able to fly for a carrier who operates under 135? I work in the corporate aviation, and at least here in Europe not many private jet operatos do have turboprop planes!
This is not correct. If you take the 121 exam, you can still work in 135 or 91... If you take the 135 exam, you can still work in 121. Corporate aviation usually operates under part 91 or part 91k. An ATP is an ATP, there is no distinction between someone who takes the 121 exam or the 135 exam.
For what it's worth, there are 121 operators who operate turboprop and there are 135 operators who operate jets.