All sound reasons, here's my take on it.
Both are lightweight 3-1/2" barrel .40's, single stack, TDAs, & of course have the low durability black finish
in common.
4040PD -vs- 4014:
---------------------
Small frame -vs- large frame.
Al/Sc alloy -vs- Al alloy frames.
Single sided ambi -vs- dual sided.
Dual nested -vs- single recoil spring (except R4 version).
25oz -vs- 27oz.
7+1 -vs- 8+1 rounds.
Both made for only 2yrs: 2003-2005 -vs- 1991-1993
The 4040PD looses (points) on common parts availability as it has no real siblings. It's pretty much a one of a kind (for good or bad).
Unique only to the 4040PD parts (not shared with any other 3rd Gen):
- frame,
- slide,
- grips (factory Hogues)
- barrel
- guide rod assembly
- slide stop assembly (shorter arm to contact mag follower)
- magazine (body, follower, butt plate)
The 4040's mags are more restrictive.
Since the 4040's mag comes from a 9mm I find it limits COAL noticeably compared to the 4013/14's.
I've always loaded my 40S&W to 1.135" (max COAL"). These won't feed at that length in the 4040 mag.
1.120"-1.125" feeds okay with most JHPs in the 4040 mag. And bullets with round nose-flat points seem like they need to be a tad shorter.
Maybe mine's weird but grouping in the 4040 is nowhere as good as most 3rd Gens, certainly not my 4013. I'm still trying to find a load that makes it look normal.
If I had no other pistol & I just wanted something for general purpose use I'd go with the 4014.
If you just wanted unique, go with the 4040PD.
.