NSString property: copy or retain?

Let’s say I have a class called SomeClass with a string property name:

@interface SomeClass : NSObject
{
    NSString* name;
}

@property (nonatomic, retain) NSString* name;

@end

I understand that name may be assigned a NSMutableString in which case this may lead to errant behavior.

• For strings in general, is it always a good idea to use the copy attribute instead of retain?
• Is a “copied” property in any way less efficient than such a “retain-ed” property?