Bronze or Cast Iron Valve Guides?

It has been awhile since I have posted or even checked Z31p, but I was hoping to get some advice or opinions on an issue.

So, I have some heads that I am working on that have some cracked valve guides. Should I go with bronze or cast iron? The heads are going on a turbo build with some mild performance. End goal is to land somewhere between 300 and 400 hp.

What do you all think? Steel valves with bronze or cast iron guides?