Trackday tyre question

[MrHanky]

Hi,
 
Just wondering what tyre size to go for on my new (2nd hand) Enkei\'s.  They are a 16" 8J wheel (to go on my EGI).  One tyre place has said that I should go for 225 / 50 / 16 and the other 225 / 40 / 16.  Im inclined to go for the 50/16 all round but just wanted to double check?
Also, has anyone any experience of the Hankook Z212?  Looking to get these as they seem pretty good value for money.
 
Thanks
 
Mark