a) Type “Save money and wait longer” = buy on site
Prices are usually a bit cheaper locally in Rome, so you would save money that way. However, you have to arm yourself with water, reading material and patience for the major sights such as St. Peter’s Basilica or the Vatican Museums.
At the Galleria Borghese it can be that there are no more tickets left – you can only get in with a reservation and only 360 people are admitted every 2 hours.
There are several queues in the Colosseum: one in front of the security checkpoint – this applies to both types of visitors and one in front of the cash desks – i.e. for you.
NOTE: Due to Corona, you can currently NOT buy tickets on site.
Of course, the queues also depend entirely on the season. The busiest months are March-April and July-October. Since the climate here is quite mild, Rome is often used as a destination for Easter and autumn holidays, but summer is also very busy!
Bottom line: You save money, but you have to expect waiting times at many sights.
b) Type “save time and spend a little more money” = buy online
Online prices are sometimes a bit higher. The museums charge an extra fee and so do the suppliers who offer these tickets.
But you get something for that! You can pass the people waiting! So, you tend to see more of the city by saving 2.3 hours of “lifetime” here and there.
Here, too, you should plan ahead for the months of March/April and July – October.
Bottom line: You might spend a few euros more, but save a lot of time at the admissions!
YOU must now decide which group of people you belong to.
But maybe a combination of both is possible. You don’t have to buy an online ticket for everything!
A mix of planned and spontaneous visits is always best anyway!