Wheely bug - they are GREAT!! For a 13 month old definitely get the small one. We were given one for DD1 for her first Christmas (she was 11 months), by her first birthday she was really into it, and it's been popular ever since with her and her sister. DD1 and DD2 (they are now nearly 4 and nearly 2) will both still fight over who gets to whizz around on it.