external drives and caddy question

Hi people
I'm looking to get another drive as my WD MYbook is getting long in the tooth and I need back up more stuff.
First I guess I'm still not sure what my best option is regarding internal or external?
What are the pros and cons? I wont initially be running samples from the drive but would like to have that as an option should I wish in the future so that maybe be a deciding factor.

My initial question was this, I have an eSata storage caddy that is Sata-11
and the drive I'll be getting will be a sata-111 drive.
Is there any point in using that caddy since it can't match the speed of the drive that I'll be buying?
Secondly, if I'm going to get an external drive am I better off with another mybook rather than an internal drive in a caddy?
I'm looking at the new WD Mybook USB 3
(http://www.scan.co.uk/Pro...5-External-HDD-USB-30)

The cheaper option would oviously be to get an internal drive but is it the best option?

Cheers
Steve