As a former Congressional Intern who led plenty of tours let me provide my $.02.

The advantage of a group tour is that you get a guide who actually knows the history of the building and Congress cold because s/he is doing this many times a day for many years. The downside is that instead of the tour being just your family and possibly other people from your state/district you are one of, IIRC, 30 people on the tour and have to go at the tours pace.

The advantage of the office tour is that is will most likely be a small group; almost all of mine were just the one family YMMV depending on the office; you will also have the opportunity to go through the tunnels and possibly ride the subway instead of just going through the Visitor's Center (although this might have changed). The disadvantage is that your tour guide will most likely be either an intern who is there for 4-6 months or a very junior staffer who will know a lot less about the building than the group tour guide will.

Even if you take the group tour stop by your member's office to pick up gallery passes if either House is in session and you might get to meet the member if s/he is around and in a good mood.

Hope this helps and don't forget if you want a White House tour you need to go through your member's office unless you know a WH staffer.