In ScriptSig, which form of the public key is considered canonical? In the standard pay-to-pubKeyHash scriptSig, which form of the public key should be used. My understanding is that both the compressed (33 bytes) and the expanded (64 bytes) are both valid. 

Is there any of these two that is considered "standard"? Will the reference implementation propagate transactions containing either form, or will it withhold one?

IMO, I should use the compressed version since it will result in smaller transactions.
Both are valid and accepted as canonical. They are NOT interchangeable however - the same public key in compressed form is hashed to a different address than that in uncompressed form. You can only use compressed form if the unspent output address is compressed.