From a pure build quality standpoint, the Razer is of MUCH HIGHER quality. Razer really makes very good quality products.
They both have similar functionality.
I would get the Razer as it is a more conventional and good looking keyboard. But I would suggest you get the one that you want. You will be happy with either. If you like something to look at the Razer wins. If you like the small form and the small steelseries touches, then go with it.
Your choice for sure, but for me SS 6Gv2 all the way. Me and three of my friends have been using the 6Gv2 for three years now (with constant pounding when playing and traveling to local LAN events ) and it is great, in terms of quality and performance definitely the best I have even owned. That said I would agree with the post on macro keys as I have heard the Blackwidow has some extra options but you will pay extra for that. You need to make sure the EULA for the game allows you to use the Macros if you are playing online.