The FAA ATP exam is **** easy. Compared to Australian & UK exams it's a couple or few days with Gleim &/or Jeppesen and maybe a day or evening with a CBT question bank - unlike UK (now JAR) that requires as much as 6 *months* of full time study to pass their 14 exams or even Australian that requires the same number of weeks to have a reasonable chance of passing their 7 or so (Disclaimer: it was a long time ago and only four exams covering mostly similar content when I did mine although there have been some content additions since). I speak from experience about those overseas exams. The cost for them can really add up so given the simplicity of the FAA exam I find it hard to understand why someone would spend hundreds of dollars for what can be studied for only tens of dollars, if it's not needed?
121 or 135 are about equally easy. If you currently fly 121, then opt for the 121 exam. If you fly 135 then take the 135 exam. You should already be familiar with the relevent rules if you operate within that environment so you won't need to study as much. Weight & balance is hardly PhD stuff so which ever exam you choose it's all much the same. It really doesn't matter because the method is the same. If you know how to solve W&B problems (like you're supposed to
) then it doesn't matter what the underlying airframe is - plug in the numbers given in the question stem and derive the answer.