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.
Some of the slower drives are cheaper. Would you use a slower big drive or get a fast big drive?
just pick a USB 3.0 drive, it is much faster.
also, software is important too.
Box+ hybrid backup allows you to back up to external drive for free. it is very much click-free like backup, once you plug in an external drive, it asks you if you want to use it as a backup drive. Once you answer yes, it remebers it and continousely and automatically backs up all changed files in selected folders to the external drive.
you can use it absolutely for free. most others charge $30-$60 for the same functionality. you can turn any external drine into a automatic backup drive