What is the reason to go w/ stainless steel valves over brass?
Brass valves contain a small amount of lead, which can leech in to your wort...while stainless is a more durable metal and does not contain lead.
Here is an article in BYO magazine about various metals and how they effect your beer: http://www.byo.com/component/resourc...or-homebrewers
All brass fitting sold in Cali are 'lead-free' so in my opinion there is no difference here, except you probably won't find any 3 piece valves in brass. The brass ones even use a stainless steel ball, with Teflon bushings, just like the stainless.