In the console.log(prop, ": ", person[prop]);

The majority of the time I use console.log, I am logging string values during debugging scripts, so then it is only a cosmetic difference when you use comma syntax over plus expression. The difference is the that the commas impose a space character between items. (See below "Henry David Thoreau example).

The reason one might choose a comma syntax is to keep the discreet (unchanged) values in the log when combining similar data types. Note three integers x, y, and z, are computed when you use a plus because of the datatype coersion, and when they are logged with commas, we can still see their original values.

> var first="Henry";
> var middle="David";
> var last="Thoreau";
> console.log(first+middle+last);
HenryDavidThoreau
> console.log(first,middle,last);
Henry David Thoreau

> var x = 0;
> var y = 1;
> var z = 2;
> console.log(x + y + z);
3
> console.log(x,y,z);
0 1 2