In my opinion, you want to use the same Apple/iCloud/iTunes ID on both devices. That is exactly what iCloud is designed for - to keep devices with the same Apple ID in sync. Your Mail, Contacts, Calendars, Reminders, Safari Bookmarks, Notes, Photo Stream photos, and Documents will all be kept in sync if you have those options set up on both devices under Settings>iCloud.
Unless you have some reason to not want to have the same information on both devices, then going with a single ID on both is the way to go.
I have 2 iPhones, 2 iPods and an iPad and iPad mini running off the same iCloud account. It means any of the songs, apps etc can be downloaded to all the devices. Back up uses the device info to keep it separated from the other back ups. iMessages would go to all devices, if you have that turned on on all devices. I just have it on my iPhone.
Thank you, this helps a lot because I do want all apps shared, etc. I have noticed however that the "Notes" app has not merged properly. That is, some notes I added to my iPad have not shown up on my iPhone and vice versa. I use Notes a lot especially for logins and stuff like that so it is pretty important. I'm not certain how to fix this but thank you for your suggestion to keep the ID's the same. My iPhone 5 and iPad are currently the same but I wasn't certain it was the best idea but you cleared that up for me. Thank you!