The 340pd and the 642 dimensionally are nearly identical.
The significant weight difference is due mainly to the titanium cylinder.
Both have aluminum alloy frames, although S&W cleverly markets the 340pd as "scandium". In reality, it is an aluminum alloy with a trace amount of scandium (very expensive) added to it to make it much tougher.
A three ounce weight difference can be noticable and desireable, especially with pocket carry. Most people will not have any problem carrying the slightly heavier one.
The 340pd costs about twice as much. The 642 is a much better buy for the amount of money spent, but at the same time, there are a lot of carry guns in excess of $600, so the dollar amount by itself is not a reason to not get one if one can afford it.
The 340pd is a .357 magnum, but it is not mandatory (or desirable for most) to shoot .357 in it. Some will say it is pointless to have if you are only going to use 38 sp in it, but it does have a weight advantage.
Shooting 357 in is NOT for those with limited shooting experience. Some will describe how horribly painful it is. Yes it stings a little and is not pleasant, but five shots isn't going to shatter the bones in your hands. The real trouble with it is recovery shots. There is a lot of muzzle flip.
Shooting 38 through either one feels about the same.
There was a limited edition called the 340ss and should soon be model called the 340m&p. These are virtually the same as the 340pd except they have stainless steel cylinder which adds a couple ounces to the total weight and that amounts to what is basicly a 642 that can shoot .357mag. The weight advantage is lost.
There are varying opinions. The cost is the single biggest factor. The 340pd does everything the 642 can do and is capable of much more even if this capability is not used (fire .357).
There are some other less significant differences between the two, like sight options. The two models have different grips, although this is easily changable if desired. And even though it is a really poor reason, everyone has a 642 and few have a 340pd.