If you plan to visit the Vatican Museums and the Sistine Chapel in the busy season you may like to prepurchase tickets to avoid long lines.
Normally worst days for linewise are Saturdays and Mondays since on Sundays the Vatican Museums and the Sistine chapel stay closed (except the last Sunday of the month when the’re open and free). Though it is sometimes difficult to predict lines.
To prepurchase tickets I always recommend the official Vatican website (www.vatican.va ) where you can buy your admission tickets (http://biglietteriamusei.vatican.va/musei/tickets/do) from 60 days before the date required. Sometimes tickets are sold out, but do not get discouraged, try again some days after since other tickets are often avaialble.
With other companies offering the same service tickets will be more expensive.