Well my 32GB iPhone has less than a GB remaining. And my 16 GB iPad has about 3 or 4 GB remaining. I'm assuming it won't fit on a 5GB iCloud account.
You can get a preview of just how much space you need for the backup, and make adjustments if necessary, at: Settings > iCloud > Storage & Backup > Manage Storage. Select the device (eg. "My iPhone"). Under Backup Options you'll learn your Next Backup Size, and can choose to exclude various kinds of data from that backup. (iOS, apps, and iTunes Store content is not kept in the backup, which substantially reduces the space needed for backup- apps and iTunes Store music are automatically re-downloaded if you need to restore from the backup).
The biggest single component of most backups is Camera Roll. Most people just keep accumulating photos right on the iPhone, never deleting a one. Between junk and images transferred to your computer (Photo Stream can automatically save every shot you take to your computer), how much do you really need to carry around with you?
I primarily depend on iCloud for backup (occasional iTunes backups are made, too). Why? I don't have to remember to do it. I stand a much better chance of being covered by a recent backup.
There are several other benefits to iCloud backup. Perhaps biggest is that you can restore from backup no matter where you are. You also have the benefit of any off-site backup - you can lose everything in a fire, and still get your data back.
By all means, backup to iTunes as well. You can't have too many backups. But why have iTunes be your only backup, when you have a free, automatic backup service at your disposal?