There is no difference in functionality, and if your client ever gets big enough when it matters, you can always contact AWS and change your account settings.
IMO, your client does not risk anything, just setting up a personal account to get started, and they may never have to switch it - just make sure that the person creating the account is not the one who is likely to quit and take an account with you or keep it for blackmail - the credentials of the root account must be controlled by the owner or other company official. All other users must have IAM accounts.
source share