PSD may once have been considered the more "native"/modern Photoshop format, but no longer. Jeff Schewe (the Photoshop Guru's Guru) advised way back in August 2007 on the Luminous Landscape forums that choosing TIFF over PSD was his strong recommendation. I quote:

Look, I'll make it REAL simple...

TIFF = Good
PSD = Bad

Here's some more detail from that forum posting, but I encourage you to follow the link and read the rest of it:

PSD is now a bastardized file format that is NOT a good idea to use. Even the Photoshop engineers will tell you that PSD is no longer the Photoshop "native" file format. It has no advantages and many disadvantages over TIFF.
TIFF is publicly documented, PSD is not. That makes TIFF a preferred file format for the long term conservation of digital files.
And, let me be blunt, anybody who thinks PSD is "better" than TIFF is ignorant of the facts. If Adobe would let them, the Photoshop engineers would tell you to quit using PSD.

The short answer is: save it as a TIFF.

PSD may once have been considered the more "native"/modern Photoshop format, but no longer. Jeff Schewe (the Photoshop Guru's Guru) advised way back in August 2007 on the Luminous Landscape forums that choosing TIFF over PSD was his strong recommendation. I quote:

Look, I'll make it REAL simple...

TIFF = Good
PSD = Bad

Here's some more detail from that forum posting, but I encourage you to follow the link and read the rest of it:

PSD is now a bastardized file format that is NOT a good idea to use. Even the Photoshop engineers will tell you that PSD is no longer the Photoshop "native" file format. It has no advantages and many disadvantages over TIFF.
TIFF is publicly documented, PSD is not. That makes TIFF a preferred file format for the long term conservation of digital files.
And, let me be blunt, anybody who thinks PSD is "better" than TIFF is ignorant of the facts. If Adobe would let them, the Photoshop engineers would tell you to quit using PSD.