I can't decide if it will be best to do the WoI tour or get the Roma
Pass to tour the Colosseum on our own. I understand the Roma Pass also
includes bus passes.
Horsewoofie, I would not buy that pass if all you intend to use it for is the Colosseum. Yes, it includes buses and metro (not to or from the airports) but we've used public transport VERY little on our several trips to Rome. It's a great city to walk and you'll experience more of it on your feet than from behind glass. Should you want to take a couple of hops on transport, , just buy a couple of single-ride BIT tickets ( €1,50 apiece) at a bar or tobacco shop and tuck them in your pocket for that opportunity.
http://www.atac.roma.it/page.asp?p=229&i=14&lingua=ENG
But here's the fuzzy part of your question: you mention only the Colosseum but not the Forum and Palatine? If you want to see these other two pieces, I might consider a tour, such as WOI offers, which includes all three as the latter two are more complex to understand without a bit of advance reading (which is how we did them).
If all you're interested in is the Colosseum, I'd look at booking the VERY economical 45-minute tour offered on coopculture, the official ticket outlet:
http://ecm.coopculture.it/index.php?option=com_snapp&view=products&catalogid=08F1776A-4A9B-B682-0EB2-01651506A87E&snappTemplate=template3&lang=en
You will have to choose a specific day and open time slot. Ticket price is € 19,00, which includes your general entry ticket to the Colosseum + the tour + entry to the Palatine and Forum (no tour) + reservation fee. After your tour of the Colosseum is over, you're free to spend as much time in the general-public areas of the arena as you wish before moving on the Forum and Palatine, or skipping the last two completely if they're not of interest.
The tour does not include the underground or Belvedere but you can see into the underground from above and you'll get the same information as visitors taking that tour.