Comparing UBot Studio and ZP (and Botchief lol) is like comparing a truck and a station wagon (and a wheelbarrow). They are all vehicles that can carry stuff, but otherwise they are very different.
1) There's a lot that we do differently from Zennoposter - but telling you the differences won't help much unless we know your plan. UBot is extraordinarily powerful and ALSO easy to use, and relies on some simple concepts that you can expand on to create a huge automation suite.
2) The real question is, what are you actually planning on automating? No one software is going to be perfect for anyone (although I'd argue UBot Studio gives a LOT).
3) It's up to you. A lot of people have both. You shouldn't need both to begin and certainly not until you've mastered one.
That being said, PM me and I can give you some more specifics.
Seth