We're going to Rome in a couple months, and would like to take a guided tour of the Colosseum's underground section. It looks like the "official" tours are run by Coopculture (http://www.coopculture.it/en/events.cfm?id=6) and are remarkably affordable (9 euros or so, plus admission) and available in English. I see also that there are lots of companies, like Dark Rome and others, that also do tours of the underground and third ring, but charge something more like 90-100 euros.
What's the difference? is there any difference in access to areas? Any difference in quality of tour guides? Size of group? Etc. We're a family of four on a budget, so the price difference is a big deal for us. But we are history nuts, so if this is a splurge that's worth it, we'll try to make it work.