I think I will have only time for one. I will be seeing the outside of the colleseum, and St Peter's Basilica with my tour group. Do you recommend The Colleseum/forum or Vatican Museums/Sistine chappel?
Vatican Museums/Sistine Chapel.
For me it would be Vatican and Sistine, but I love Renaissance art. Which is more important to you-Roman history or Renaissance art?
Two completely different sites. My favourite was the Vatican; not for the art but because our tour guide was so good at describing religious history and the story behind the art. I also loved the Forum/Palatine Hill/Colosseum. Of those 3, the Colosseum was the least impressive from the inside. As with the Vatican, I'm glad I took a tour or all I would have seen was piles of rubble.
I did them both in one day! It was my first full day so I made good use of knowing I'd be up and about by 4 am and started with a 6:15 metro ride to the Vatican museum where I had a ticket for a small group guided tour that started at 7 am--those 2 hours before the museum opened were blissfully peaceful, and the ticket allowed me to stay as long as I wanted after the tour ended--but I left by 11 am and explored the area near the Spanish Steps and had a late lunch before heading to my hotel (I stayed at the Lancelot which is near the Colosseum) for a short rest before my 5:45 entrance to the Colosseum. That ticket allowed me to also visit the Forum and Palatine Hill on a separate day, which was perfect for me.
I suppose if I really had to choose, it would be seeing the Colosseum from the outside and touring the Vatican museums.