Plastic top or metal tops better?
 

I just came upon a couple of pics:

http://www.radiical.com.au/Images/Aluminium-top.jpg
http://www.radiical.com.au/Images/Aluminium-top2.jpg

Are all metal tops susceptible to this?

Does it goes on to say that plastic tops are actually better?

It all depends ...
Plastics tops (at least some of the commonly used types of plastic), tend to crack every once in a while...
Some metal tops won't have the same problem (brass for example, or decently anodized alu even)
And a lot of what you see there (galvanic corrosion) can be avoided by using additives in your fluid.

My preference goes out to brass tops ... no cracks, very little corrosion, cheaper than copper

generik, that's corrosion caused by having aluminum in the loop. If you avoid having aluminum in the loop, use enough mix of zerex/anti-corrosion agent, and flush the system when needed, it won't happen.