8 ounces different? So you have it narrowed down to a Kahr PM40 or an MK40, I suppose (correct me if I am wrong).
The question here then isn't about stainless per se, but rather a metal frame versus polymer frame. Overall differences have been covered here at THR many times, but I will outline them again for kicks:
Polymer Frame:
+ Lighter and easier for pocket carry
+ Lower maintainence
+ Black, so more difficult for bad guys to see
- Higher field recoil
- The "Tupperware" factor (can feel cheap, as some report about Kel Tec)
- Many consider it uglier
Alloy:
+ Reduced felt recoil due to increased weight
+ Feels like a "Real Gun" in hand
+ Can be real pretty - just see the MK9 pic earlier in this thread
- Cumbersome to pocket carry - many claim they are only suitable for IWB save for Seecamp and Guardian and Rohrbaugh
- Can be more difficult to keep clean (i.e. prints on blued or coated finished), or requires delicate care to maintain (i.e. the 3m Scotchbrite pad)
-If stainless, can give up element of surprise due to shiny finish.
This are of course all opinion, but do seem to be the prevailing opinion. People's perceptions can vary. For example, I consider polymer easy to clean (wipe down), and stainless a pain to scrub. Oro, on the other hand, had the opinion that stainless has "reduced maintenance requirements." We can probably both agree that either are easier to care for than a fancy blued finish, or a coated finish (Tennifer or Melonite) than can chip and scratch. You just have to come to your own opinions and determine which priorties are more important for YOU.
For me, I have a polymer service pistol (XD), as I like the reduced weight on a gun with a full 4" barrel and 12-round mags. I do want to secure a metal-frame pistol for IWB concealed carry (2075 RAMI or MK, either in 40), as I gain the advantage of reduced felt recoil on the smaller but denser weapon.
So each frame material has its place. Hope this helps