Nino, I bet you are now more confused than you wre when you started.
This is why I don't usually answer questions like: what do you think is the best...... or what type of fish should I have, etc. etc.
You will have a multitude of answers and everyone will feel that hey have the best of..........(whatever)
You need to set down some rules before asking a question that will inevitably give you a multitude of answers. Like a search engine you need to narrow your parameters to: What you want it for including size of tank, how much you are prepared to pay for it, the type you want,ie internal or external and the amount of time and the space you can afford the unit and what job youwant it to do.(that's usually the hardest question)
This gives the reader something to work with.