Rome

Do both. Neither one is going to consume a whole day. And they are not that far apart.

A Coliseum tour would seem to me a waste of time. The outside is impressive enough.
I can't advise you on Vatican tours. I'm not a tour person.
I would nicely spend the day walking through the city, have a look at a church you fancy, admire the fountains on the Piazza Navona, have a good look in the Pantheon, spoil yourself with a good lunch, have a look at the Forum, pick another fine church, and look for a place where you can have a coffee, beer or wine to look back at a relaxed, well-spent day.

Judy, you probably already know this already but your one day wouldn't happen to be a Sunday and/or a first Sunday of a month? There would be some complications with that but will only fill you in if that's the case.

Have no idea why a coliseum tour would be a waste of time. The insides are just as impressive as the outsides. I like live guides (over printed walking tours) because you can ask questions. I would arrange a morning tour - generally cooler - and hit the Vatican in the PM.

Assuming it is not a Sunday. . . I suggest you book a tour with early entry to the Sistine Chapel, followed by either the Vatican Museums or St. Peter's, depending on your interests. Or you can do a tour that includes all three but personally I would be "museumed out" by that.

There are several companies that offer the early entry; one is Pristine Sistine with Walks of Italy (10% discount if you mention Rick Steves); we used Dark Rome which had exactly what we wanted.

I don't know Bout the "official" tours of the colosseum as we had a private guide for that. Actually the Forum is far more interesting if you care about Roman history. You get a combined ticket for both, plus the Palatine Hill. I suggest you do the Vatican tour of your choice in the morning, then book a "skip the line" ticket for the Colosseum/ Forum/ Palatine Hill and go in the afternoon, using the Rick Steves audio guide. Start with Palatine Hill (least crowded) , then Forum, then Colosseum last after 3:00 when crowds abate.

Well, they're pretty different from each other, as you can imagine, so it depends on your interests. That said, the Vatican tour would have a lot more substance to it, as it will cover lots of different things. The Colosseum is one note (albeit one huge note).

Didn't take us all day for a Coliseum tour...we did the 3rd ring/below and it was (excuse my bad memory - it was last year) maybe 75-90 min. You could throw in a walk around Palatine Hill/Forum - that could take another 60-90 min depending on how indepth you want to take it.

We did a Walks of Italy tour (after hours tour) at the Vatican - it was about 3 hrs long and touched on the highlights. But I think you could spend the whole day in there on your own to see it all.

I guess do you want to see art and treasures (Vatican), or architecture and ruins (Coliseum)?

If i only had one day in Rome I would walk around, sit in cafes, duck into any church I happen to pass by, eat, walk some more, etc. For me that is what i love about Rome. However, I have been often and have seem many of the sites.

Join an early morning tour at the Vatican, it's pricey, but worth it. It will hit the high points and you won't have to spend energy figuring out what to see. Easy to spend the whole day, you can walk up to the top of the dome, spend time in St. Peter's Basilica and Square. Or do the tour (ours was 3 hours) leave around lunch time, find a cafe to eat on the way, then head to the Coliseum and do the Rick Steve's tour that you can down load to your phone/mp3 player. An hour is all you need on the inside, but well worth it to look around inside. We didn't do a formal tour there.

I was just there in September and did the Walks of Italy pristine sistine and VIP colosseum underground.

If I had one day I would choose only one! As much as we loved the tours, we enjoyed walking around to the Pantheon, Piazza Novanna, Spanish Steps, and meandering the shops and restaurants even more.

They are both wonderful tours. I was absolutely exhausted after the Vatican and that is the only day I had to go back to my hotel room my back hurt so bad and I was just tired. I am in shape. So for me, I would do the colloseum, forum and palatine hill tour...then go walk around Rome.

DH wasn't tired in the least after the Vatican, and loved it more than me.

We just returned and I can't imagine seeing the Vatican without a tour. The lines were CRAZY -- plus, there is SO MUCH art and history, you will be overwhelmed on your own. Tours will point out what's important to see and why and what you can walk by. The museum alone is 4 miles of art. Not to mention the Sistine Chapel and St. Peters Basilica (which were our favorite parts of the Vatican Tour --we were fortunate to be on a RS tour). Since we were on a RS tour, we also had a guide for the Colosseum & the Forum (don't miss the Forum and the Pantheon). Rome is full of history everywhere you look. One day is going to be tough. A tour will get you through much faster. You could definitely do both w/the right timing - have you thought of hiring a guide to take you to all 3? I know the RS Italy book has private guide recommendations.

If you have only one day in Rome, and have never been before, then I would walk, walk, walk ...
Start at St Peter's Basilica (skip the Vatican Museums)
Head via Castel San Angelo, crossing the Tiber to the Pantheon and lunch by Piazza Navona
Then the Spanish Steps, Trevi Fountain and Vittorio Emanuele monument
Finish with sunset photos at the Colosseum

If you can, get an early morning tour of the Vatican Museums and Saint Peter's Basilica (pricey but worth it); this should be over by 1pm, then take an afternoon tour through the Colosseo and Foro Romano, either guided or on your own. If you have a chance to go to the Pantheon or to San Luigi dei Francesi, these are close to each other.