If you take the benefit as salary your tax liability will definitely increase since you will no longer be able to claim the housing exclusion. There's also more....From what I've read there was recently a new ruling within the IRS that from 2006 onwards the expat housing exclusion will be cut. If this is true, it's a big blow to all those expats (which included me last year) who've counted on housing allowance as a nice benefit, and will significantly increase tax liability no matter which way you get the allowance (as salary or housing benefit). This is expecially true in HK where housing is so expensive. Read this article in the Washington Post:
By Daniel J. Mitchell
Special to washingtonpost.com's Think Tank Town Wednesday, June 28, 2006; 12:00 AM
The U.S. is one of only a handful of countries that insists on applying an onerous system of "world-wide taxation."
America's policy makers have tried to mitigate the adverse impact of world-wide taxation by exempting Americans living overseas from paying U.S. taxation on up to $82,400 annually. This is the "foreign-earned income exclusion" in Section 911 of the U.S. tax code. Thanks to a last-minute amendment inserted into a recent comprehensive tax bill, the foreign income exclusion will be slightly raised, but other benefits, such as housing exclusions, will be cut -- resulting in a huge spike in tax payments for many American expatriates.
Now, this is just what I've read so it's not confirmed and I advise you to get the "real" details from a tax attorney.