Ciao!
When are you visiting Siena? You can buy the pass beforehand, there is only a 1 euro booking fee so it isn't a lot. I've been to Siena in the summer and sometimes there is a long line at the ticket office but I also saw it moved really fast. So you don't have to buy it ahead of time but could be useful, if you have limited time in Siena to start with.
The OPA SI pass includes the cathedral, baptistery, crypt and museum... if you're interested in only the cathedral, there is a ticket just for it as well. But the pass is a good deal if you're planning to see anything else or are there for an entire day, as you should definitely visit the other sites as well.
The OPA SI + card also includes the visit to the "Door to Heaven" itinerary which is a guided tour and takes you onto the roof of the cathedral. This costs more and you should book ahead of time, since the tours are for specific times every day.
Inside the cathedral, your visit is on your own. But they do also offer guided tours in various languages. You request them when you buy your pass, even if you buy it there on the spot... but they'll tell you when the next one starts and in the meantime you choose what else to do/see.
They also offer a self "guided" tour with a tablet installed with a special app that you can rent during your visit which has additional explanations and photos to explain what you're seeing.
So there are various options as to what to do... it really depends on how much time you have overall as to what ticket is best, and whether you would like the visit with a person or the tablet. I personally have always found the tours by people specialized in the place to be excellent, as they also answer your questions!
