What is the difference between the two batteries?

The primary difference between the main battery and the PRAM battery are their size difference and function. The main battery is responsible for powering the entire computer. Considering the age of your particular model of mbp replacing the main battery would be advisable if you want to avoid being tethered to a wall outlet whenever you use it. The PRAM on the other hand is a small lithium battery or clock battery and it's used to maintain the state of a small amount of memory which contains certain system settings including the system clock.

The standard symptom of a failed PRAM battery is that your machine's date and time will be incorrect when you boot your Mac e.g. you'll get a message at restart that reads "Your computer's clock is set to a date before March 24th, 2001" - it will generally correct itself shortly after boot when your Mac checks an internet time server. In some Macs you will also see power management related issues if you have a failed PRAM battery.

If you don't have any of the above issues you probably can avoid tackling the replacement of the PRAM battery altogether. The average lifespan of a PRAM battery can vary, but if you use your computer on a daily basis, battery life approaches shelf life... I've seen them last as long as 10 years.

Just replace the main battery.

Prost