When you use the plain text link it will use the default checkout method. 

If you want to give them the choice then it needs to be via the embedded form so the system can collect the needed info. 

A backdoor hack would be to append &u_payment_gateway=paypal onto the end of the URL: 

http://nanacast.com/s/?req=form&cue=subscribe&u_list_id=12345&u_payment_gateway=paypal