A TC is the easiest to setup and run and is officially supported by Apple. Others like NAS devices, but support can be spotty and OS updates can cause them to stop working until the NAS vendor updates their firmware.
The TC can be used to backup multiple OS X devices, but it will not directly backup an iOS device. What you need to do is plug the iOS device into one of your Macs then use the manual backup option in iTunes. That will create a iOS device backup file on the Mac which is in turn backed up to the TC.
Apple has refurbished Time Capsules
here for $44 off MSRP.