Could someone please compare/contrast these 2 Rome tour companies as to quality, price, tour guides, tour size etc. I will be in Rome in October and want to take a couple of tours (either the Colosseum, Vatican or city walking tours) Thanks.

This has been a popular topic on this forum recently. Have you done a search using "scala reale" and italy? You'd find a wealth of information about Scala Reale and other tour organizations. Here's one such string: http://fodors.com/forums/threadselect.jsp?fid=2
Good luck.

I only know Scala Reale:

small groups (6 or less)
at least 50E per person (haven't checked recently) though I think there are family discounts

guides are history/art/architecture scholars or teachers, both knowledgeable and entertaining

Tours are 3 or 4 hours

Quality excellent, imo.
And customer service excellent as well.
Very efficient bunch, live people answer their phone even into the evening.

Sorry, that link is no good, but the search information is accurare.

I meant to add
I've read only good things about Through Eternity
Don't assume there's only one "best"; just as in hotels, there are in a large city likely to be several great alternatives