Bonnet mystery

[Don McLellan <dmclella@netrover.com>]

Hey folks...  I've got a question that most of you probably already know 
the answer to.  I have a 70 MGB with an aluminum bonnet.  It's kinda 
banged up at the front and has been puttied (or should I say 
sculptured).  So my friend wrecked another 70 and has been storing all 
the parts for a few years, and brought over a pristine STEEL bonnet for 
me to install.  So what gives?  What dictates that a car would get 
either a steel or aluminum bonnet?
I gotta know!!!
Thanks , Don