Spark plugs

What are people's thoughts on what type of spark plug to use during tuning. My coils are from a Honda CBR to suit a M&W CDI and require a plug that has removable tip. I can only find platinum and iridium plugs that have this. I'm not sure if it is possible to machine the solid tip from a copper plug without breaking it, if I was to use a copper plug.

What do you guys recommend to use?

IF your working with CDI the I'd have said it's best to stick with Platinum and Iridium plugs as they are a hardy plug and will last with the energy going through them, I only use copper plugs on weaker ignition systems as they have a better conductivity and will have less energy passing through them.