Difference between "site:current-user:field-foo" and "account:field-foo" in setting a Rules condition? What is the difference between "site:current-user:field-foo" and "account:field-foo" in the Rules "Add a new condition"?
As I mentioned in the comments the variables you have available to you in a rule depend on the context it's in.

[site:current-user] is a global token available to any rule and will always return an object relating to the currently logged in user.

The [account] token is there because you're responding to the "After saving a new user account" event; that event passes an object relating to the user object that has just been saved (for obvious reasons).

I guess the main distinction is that the former will always give the currently logged in user, while the latter could be a different user object altogether (if, for example, an administrator had created the account).