Apparently the Argos £4.99 ones aren't burst resistant, so it's worth paying the £8.99 to get a 65cm Reebok one from Argos.
But unless you're about 5'11 or more, don't get the 75cm Reebok one which is £12.99 and just too big if you're only 5'8... gym balls are one of the items you can't return.
Guess which size I bought first...