I went searching again, and in the Luminous Landscape Forum, these folks made the case for TIFF - 6.0 layered files: "Everything that can be saved in a PSD can also be save in a layered Tiff-6.0 file format...everything, paths, channels, layers, transparency, layer effects" and that it is better for archival purposes because it's a documented format. HOWEVER...
I did a little testing in PS CS4 (yes, not current.) I can save as TIFF, but it does not indicate what version of TIFF it's using. Also, in CS4, the option to save transparency is disabled. Capture.PNG 33.25KB 0 downloads.
I added a layer mask with transparency gradient to a test file, saved to a TIFF, and and then opened the TIFF. The transparency gradient WAS NOT RETAINED IN THE MASK! The layer that had the gradient mask applied now has a solid black mask, which means that the layer does not show at all in PSD.
The good news is that the thumbnail and image in Picasa looked right. Then I reopened the file is PS CS4, now the layer mask shows. I have not idea why reopening the file gave different results, and I don't trust it yet.
I looked at the File Info for the file, and it does NOT say what version of TIFF it saves in. So at least in PS CS4, I cannot recommend TIFFs for PSDs that use transparency. :-(
IF and WHEN I can upgrade PS to the current release, I'll try it again. Hope this helps.