Slow or fast backup drive?

Slower drive: wait longer to backup, wait longer to restore when your system is down. But I would be more concerned with the interface speed (USB 2.0 vs. Firewire vs. eSATA vs. USB 3.0) than the drive speed. If your drive is external, the interface may be your limiting factor.

That's why I use hot-swap SATA drives for backups.
 
Oh, are you asking for an extrenal storage?

Then it won't mattter, as the path you choose (USB 2.0, Firewire,USB 3.0, or eSATA) will determine the transfer speed.

Except for USB 3.0, the drives will be faster than the pathway.
I never got my eSATA to work like it should, never got better than USB 2.0 speeds (10-15 MBps).

I haven't tried USB 3.0 yet, as I have no need.

My backups are internal drives, and I also backup thru a Home Network.