I'm a bit confused about all the Colosseum Ticket Options. Is the Full Experience ticket worth the 6 euro upgrade? Also, it looks like the version of the Full Experience with the underground levels and arena is especially hard to secure, and requires booking a specific time. That being said, I'm thinking to target late afternoon, but am conscious of closing times for the site and also for the Roman Forum. Since this will be late August, is say 16:30 a good time to target for the visit? Thanks for any advice!
ALL tickets for the Colosseum require booking a specific entry time (maybe you knew that?) That's how they manage crowd control...and it's still very busy! Yes, the 2 "Full-experience with Underground and Arena" tickets are the most difficult tickets to obtain; can sell out within minutes of becoming available. One is a € 32.00 tour, and the € 24.00 version is just an accompanied visit to the underground piece. You can see parts of the underground from the general areas above so I personally wouldn't put myself through the pain of trying to get hands on those and book whatever is available. You might be just fine with a general-entry ticket + some reading up on your own (€ 18:00) or the € 26.00 basic tour.
"COLOSSEUM - GUIDED TOUR" is a guided tour of the 1st and 2nd levels but does not include access to the arena, underground, attic or SUPER Sites at the Forum/Palatine. € 26.00
16:30 would be fine as long as you can get done, over to the Forum/Palatine before last entry at 6:15, and done by 7:15. The Forum and Palatine are complex sites that benefit from a tour (not offered by the official website). Morning is usually best to try and get through all three to avoid afternoon heat.