It depends on if you trust the seller and they in turn trust you. If you have seen the registration papers and sellers ID, ideally at the home where it is registered, then I'd go with cash. A cheque or bank transfer would take time to clear and you wouldn't get the car and paper work until it's cleared - possibly an issue if they change their mind about selling or if they intend to defraud you. But if you pay cash and they sign the transfer of ownership papers in front of you you can drive the car away there and then. It's up to the seller to send in the change of registration but take a picture of it, signed by the seller etc., or agree with them to post it that day yourself. Also ring your insurance and get the new car covered before you drive it away.
Cash only becomes an issue, IMHO, if you're going to meet them somewhere you could be easily robbed etc.