I am currently receiving a pretax housing benefit at 20% off the
market rate.  I now have to decide if receiving a 40% after tax
benefit is a better saving.  How do I check these numbers to see which
is better for me.

Aline --

It depends on your marginal tax rate or what you're paying on your
last dollars earned.  However, since I don't know your earnings or the
exact cost of housing, I'm going to post this as a comment.

Here's an example:
*  single person earning $50,000.  Marginal tax rate is 25% (see 2004
Tax Rate Schedules).
*  housing cost is $800 per month (market value = $1,000 per month), pre-tax
*  same housing in after-tax dollars would be $600 or 40% off.  You'd
have to earn $600 / (1 - tax rate) = pre-tax or $600 / 0.75 = $800.

It's a push here -- but obviously as the tax rate goes up, the
after-tax situation becomes more valuable.  If your tax rate is below
25%, the pre-tax housing is more valuable.

I'd suggest doing a real computation with likely earnings.  

Best regards,

Omnivorous-GA