The 3 ounce weight savings does make a difference for pocket carry especially with shorts or loose pants. At nearly twice the price, it is not justified by most people.
Many will say how horrible it is and how they can't imagine firing .357 in something so light. Some will claim their hands hurt for a week after firing one, an extremely exaggerated claim.
Yes, firing 357 is not pleasant and doing so is not for the inexperienced. The primary purpose for a 340pd is for carry, not target shooting. And it is not mandatory to load one with .357 anyway. Your hand WILL sting after firing five shots of .357. While many will be quick to complain about the recoil of the .357, there is a much more important issue to consider. Follow up shots are much more difficult. Again, this is not for the inexperienced.
The 340pd/340 will do everything a 642 can do and more. The only advantage the 642 has is the price.
There is not an appreciable difference in recoil when comparing a 340pd to a 642 when using .38sp in both. Another small benefit of the 340pd is it is a lot stronger than a 642 since it is designed to fire .357 and doesn't really matter too much. If someone wished to fire .38 +p a lot, it might matter slightly for longevity.
Another option mentioned by someone else is the 340m&p. At 15oz, it is more like a 642 that can fire .357. It will likely "hurt" just as bad as shooting a 340pd. What really defines the 340pd is its titanium cylinder, that is where the significant weight savings is. The frame is not really made of scandium; it is aluminum alloy containing a trace amount of scandium. While this significantly changes the physical strength of the alloy, it does nothing to change the weight for this application.
There are a few minor differences though, for example, the 340pd is available with a fiber optic sight. There are also different grips on the difference models, but this is something easily changed by the buyer.
Some will say the extra cost is is waste of money. There are a lot of different carry guns in the same price range. Look at the cost of a compact 1911 or a compact Sig or HK.