The biggest factor in choosing to go with sites is size. A single site
collection cannot span multiple content databases. Microsoft
recommends that a single content database should not grow beyond 200
Gigabytes. This isn’t a hard limit, and with expensive hardware (Disk
sub-system performance of 0.25 IOPs per GB. 2 IIOPs per GB) you can go
up 4 TB however 200GB is the best practice for optimal performance and
reasonable backup procedures. That being said, it really depends on
the type of site that you are designing. For an internet facing site,
a single site collection can work great. However for an enterprise
portal and document management system that will service thousands of
users and multiple departments, multiple site collections is usually
the better approach since 200GB is a drop in the bucket these days.
The biggest advantage of going to site collections is that your
content databases will scale much better beyond the recommended limit
of 200GB. What this means is that you can use multiple databases to
power your portal, which also means potentially multiple database
servers. This allows your SharePoint server to scale much better along
the data tier with additional servers.
Also of advantage is the ability to place database servers in separate
geo distributed locations to increase the performance of farms that
are accessed across multiple offices around the world.
there are other factors that can be found here: