Originally Posted by haubd

Won't help with when/how to get your tickets, but maybe helps with the decision making:

We went on a last minute (didn't decide until mid-December) trip to Rome between Christmas and New Year's Eve last year and had no problems getting Arena tickets online just a few days in advance; Underground was sold out and Attic closed.
We got tickets late in the day and were one of the last people to leave the Colosseum, so we were able to see it with fairly few people (of course, you kind of have to time it so you don't have to rush through at the end, but as the path through is oneway only, you won't miss anything.
The Arena and the walk around the Colosseum is self-guided (only the entrance time is regulated), once in you can take your time. IIRC the Attic and Underground are only accessible with guides (and you start your tour with it) and the Underground tour finishes essentially in the Arena part (the Underground ticket includes the Arena, IIRC)

Personally, I'd try to get Underground tickets, as that is a very different area of the Colosseum underneath the arena compared to the Attic, which, as I understand, is "just" going much higher up in the stands for another view down onto the arena.