If you go jointly on the loan you are jointly liable for paying back those funds from the beginning of the loan for its lifetime.
If you are going guarantor you are only called into play in the case of the loan going into arrears, if there loan is kept up to date you are not liable for anything. Depending on the amount involved you would be guaranteeing the full loan amount unless specifically specified in the loan documentation which you will have to sign.
Someone going jointly is much better security for a bank/building society I dont even know if they will accept guarantors over joint hirers any more