Trying to compare the interior of the Colosseum with the interior of the Pantheon is like apples and oranges. The Colosseum interior is much like the exterior -- by that I mean that the original facing and decoration is mostly gone. By contrast the Pantheon interior is largely intact (with a few changes and restorations over the years) because it was converted into a Catholic church fairly early.
So it's extremely difficult to compare the two. If you book a tour that might give you free time for lunch in the vicinity of the Pantheon or Piazza Navona, you can buy your own ticket from their kiosks outside the building or at the desk (both accept credit cards I think), but you will still have to wait in a line probably. Depending on what time of the year your visit is, the wait may be long or short.
Or you can buy a timed ticket in advance online -- but you may still have a wait to get in due to capacity controls.