We will be in Berlin for four days. We understand that it is a good idea to have a reservation early in the morning on the first full day to go up Reichstag. I see there is guided tour and also the dome, but there is also just the dome. What is the difference and how much time? We will be in Berlin a total of four days.
I believe a reservation is required and must be made in advance - I want to say at least 2 weeks? We did this in 2014. I don't remember having a guided tour so I guess we just went to the dome. I think we spent about an hour total.
When I went, also in 2014, I did take a tour which went into some other areas of the building including the parliament chamber, the underground museum, the walls with the graffiti left by the Russian soldiers in 1945, etc. Having a guide explaining all of this was an educational experience and very interesting. After the tour, you're left to explore the dome on your own. The tour was in English and it was free, you just need reservations in advance from their website. I do recommend the tour.
I agree with Nancy that the building tour was well worth it - very interesting and educational, and at the end you get to do the self guided (audio headset provided) tour of the dome. You must reserve a specific time slot (it's the option called "guided tour with dome"). Some days tours are available into the evening hours (I went at 6:30 p.m.) and last about an hour. Though the system currently shows dates clear to December, they only accept reservations for the current and next month or so - keep checking if your visit is a ways off. If you do get a reservation, be prepared to go through airport style security and you must bring your passport. The building tours are not available if parliament is in session, but the same reservation site allows you to reserve just a dome visit with plenty more options for entry times (from 8:00 to 22:00). Note there are dates the dome is closed for cleaning (April, July, September, October).
We also did the guided tour and thought it was one of the highlight of our Berlin trip. Norman Foster restored the building under the concept of "living history" and it's absolutely fascinating to experience it. You cannot access the interior of the parliament without a guide (reservation required but it's free as well). At the end of the tour, you get a chance to go up the dome as well where you can use an audio-guide for a self-guided tour.