Hi,
both methods should work well.
Now, if you have your account entity/collection to hand, you may prefer the second one since it does all the inner plumbings which may for instance allow you to save all your users recursively in a single call to save your account collection.
I also usually prefers userObject.Account = accountObject than accountObject.User.Add(userObject) since it does update both sides and corresponds to the natural action, though I reckon it's permitted to increment the collection since a few releases.
Cheers