[sdiy] CMOS chip questions

[David G. Dixon]

I'm currently redesigning a matrix keyboard circuit, and I've got a couple
of general questions for the "gurus" about CMOS.

First, presuming a 5V supply (from a 78L05, for example), is there any
reason NOT to always use the 74HCXXX version?  The advantages are speed,
very low power consumption, and output data levels exactly the same as the
supply.  Is there any downside which would justify/demand using the CD4XXX
version of the same chip?

Second, I'm finding that I need the odd AND or NAND gate in my circuit, and
I'm leaning towards simple diode logic (without or with schmitt trigger
inverters, respectively) to save board real estate.  Multisim simulation
tells me that this should work swimmingly.  Are there any pitfalls I should
be aware of?