HDD for backup

I am buying an iMAC and planning it's configuration.
I have a spare HDD formatted for Linux (ex3/ex4), which I will use for automatic, scheduled backups via my router. I can either buy an enclosure to use it with a USB 2.0 connection to the router or an NAS enclosure and use the RJ45 gigabit connection.
Which connection method would be preferable?
and
How should the HDD be formatted? (does MAC recognise Linux formats?)

A new iMac has 4 USB 3 ports which are backward compatible with USB 2. Connecting a USB 3 drive to one of those ports using the included Time Machine software will probably be the fastest way to backup. However, a USB 2 drive will work albeit somewhat slower.

The Mac operating system (OS X) can not read nor write to Linux formats. There may be a driver available that I'm not aware of. You can do a search for one if you choose. Any drive that's used for backing up a Mac must be formatted to HFS+. (Mac OS Journaled)

Thanks for that. It does not matter that Mac cannot read Linux discs, it's just a matter of getting prepared with the right options.