I have an iPhone 4 with an iTunes account. My husband got his own iPhone 4 today. Is it better to have one or two iTunes accounts, but not have to rebuy apps.
It's easy to break things down with the two "rules of 5".
Each account (Apple ID to be technical) can be authorized on 5 computers at the same time.
Each device is happy to track purchases from 5 different accounts.
There is no limit on the number of devices that get to sync to any computer that is currently "authorized" so hundreds of devices can feed from that one iTunes library. All purchases from the 5 accounts that are "authorized" get to sync without passwords being entered.
So if you both had an account - your shared computer would count as one authorization for you and one for your spouse. Each of you could take your iPhones to 3 other friend's house and sync apps and music there (since you used 2 "slots" for your accounts). Most people never come close to the 5 limit.
You do need to know each account's password to update the apps on the go - so having twice the accounts means an extra step to update on the road or having to wait for a password or having the apps download at home by your spouse.
There's no way to split things up later - so if you don't mind the extra hassle of two accounts (and remembering to sign in and out on the computer to keep straight which account is buying what from the computer) it's often better to have two store accounts.
I always recommend if you don't share underwear drawers, don't share iTunes accounts. There's nothing wrong with a little individual space - especially when it's easy to share whenever you feel like it.
I think that Apple's article on iTunes Home Sharing should cover what you're looking for. My husband and I have separate accounts, and anything we both want, we share.