which tag is right

We're a supplier of steel components to the automotive industry and are looking to RFID to solve some of our inventory discrepancies.

What tags are the right ones to use in a 'steel rich' environment? I've looked around and some say LF, some say UHF and some MHF.

I would like to track (atleast) input and output, but would also like to know the possibilities of tracking the whole inventory at any given time.

Would active be better that passive in such an environment?

Cheers,
Terje