One factor that is often left out of the analysis is weight. The Plus adds about 2 ounces to the Shield, which is already heavier than the 642 by 3.75 ounces. (all weights unloaded - obviously 11 rounds of 9mm outweighs either 8 of the same or 5 of .38 Spl). Otherwise dimensions of these 3 S&W offerings are similar. While I recently read 0.1 inch in width presented as justification for choosing a Springfield XD-S over a Hellcat, it wouldn't matter much to me, and that is the approximate difference between the Shield and the Plus and between the Plus and the 642.
For multiple reasons I pocket carry 90% or more of the time. I've been carrying concealed for over 40 years and I know what works for me, today. I used to be able to conceal a 4" N-Frame and two BUGS. Not feasible or necessary today. I switch from jeans and T-shirt to suit and tie, sometimes more than once in the same day.
With the 642 it doesn't matter. It carries as easily in either attire or the sweats I wear when bumming around the house for that matter. The Shield (1.0) works fine in the jeans, although the grip angle of the 642 is easier for me to acquire. In the suits the extra weight of the 9mm is noticeable. It sags, it bangs against my leg. Sure, I could use a shoulder holster under the coat and carry a pair of full size M&Ps and two reloads for each, but the coat doesn't always stay on.
Another 2 ounces curb weight plus another 3 rounds of ammo in the same pocket is not in the cards for me. On the other hand, if I didn't own a Shield already, I might have considered the Plus. Because the size and weight differences between the two 9s are so slight, a 30% capacity increase would be a real consideration.
If the local chapter of the Vagos decides to get even for what I did to their brothers 30 years ago, I'm toast. Frankly, so would I be if I was carrying my BHP and every magazine I own.
One more opinion, which is worth exactly what you paid for it.