If you absolutely are positive that you want some of them to paint up or add to your collection ( Regardless of mathhammer or any other competitive reasons ) and are going to buy some anyway, I suggest pre ordering for two reasons :
1 - You intend to buy them from your local
GW anyway so why not get them on release
2 - The entire pre order range is selling out fast, here in Aus they came up for pre order Saturday and when I checked online Monday morning the whole range was Out of Stock
Pretty much just comes down to why you want them and how soon you want them