question.... which is better??
both landline and mobile number but both small?
or
mobile OR landline number only but large? 
IMO both numbers.  I have a answer machine on the land line and the mobile is always with me.  I would not just put a mobile number as it doesnt look professional, IMO.  having a landline lets people see what area code you are in and helps to put across the 'local' element of advertising.