Yes, you can back up your iPhone to iCloud and your iPad to iTunes or vice-versa or back them both up to iCloud.
Not all of your data is backed up so you shouldn't have to worry about the 5GB limit. For instance I have a 64GB iPad and have used about 50GB's of storage. However my iCloud backup total is just shy of 1GB. That's because a majority of my files are music, video, etc. Those are already on my computer, so they aren't moved to iCloud storage. Apps and iTunes purchases aren't counted in the total iCloud storage used either. Here's a list of what is backed up to iCloud:
iCloud: Backup and restore overview
Having said that you can also backup your iDevice to
both iCloud as well as iTunes. I use iCloud for daily backups, however I also perform a "manual" backup to iTunes on my computer about once a week. Of course I backup my computer (in my case with Time Machine) hourly/daily, etc. That way if for some reason iCloud is down or something happens to my iTunes backups I can still restore my iDevices.
The name "iCloud" is a little misleading IMHO. Although it does store a backup of your iDevice(s) as well as photos via Photo Stream for 30 days, it's not a true "cloud" storage service like Dropbox.
iCloud's greatest feature really is that it keeps critical information in sync between all of your Apple devices, iPhone, iPad, iPod Touch and Mac computers. I took a photo of our dogs with my iPhone yesterday and within 10 seconds the photo was on my iPad and my iMac. It's pretty amazing.
Other features such as "Find my iPhone/iPad/iMac" as well as document storage for various apps in case you need to restore them is a bonus.
Hope that helps and here's some more info about setting up iCloud:
Apple - iCloud - Learn how to set up iCloud on all your devices. 
