My bike has Kenda Kourier tires size 700 x 40c

I've been having a lot of blow outs, particularly when running at/just under max air pressure. A trip to the bike mechanic revealed a small section of wire bead that has frayed away (OR was missing as a manf. defect) on the troublesome tire. That is likely the root cause of the repeated blowouts.
SO, my obvious next step is to replace the tire, maybe even both tires.

I've been looking at and considering several types/brands but most of the one's I'm considering are slightly different in the size. Example Schwalbe Marathon Plus is either 700 x 38c or 700 x 42c.

So the question: given the option of going slightly wider (42c) or narrow (38c) from the existing 40, which would you choose and why? Will the difference even be noticeable?