Is Mars brightest from Earth at opposition, perihelion or somewhere in between? After reading this article, I was inspired to run some simulations at NASA's JPL Solar System Simulator to see Earth's and Mars' orbits from above.

From what I could visually determine (from the JPEG images), Mars is at opposition on July 26, but its perihelion is sometime around September.

When (and where in their orbits) is Mars brightest from Earth this year?
JPL HORIZONS gives these values for Mars as seen from Earth.
r is the Sun-Mars distance in AU, delta is the Earth-Mars distance in AU, and S-O-T is the Sun-Earth-Mars angle in degrees.
I added [] to indicate minimum or maximum values.

Date__(UT)__HR:MN   APmag         r     delta    S-O-T

2018-Jul-23 16:00   -2.75   1.40246   0.38911   172.30
2018-Jul-25 14:00   -2.77   1.40106   0.38730   173.27
2018-Jul-27 12:00  [-2.78]  1.39971   0.38600  [173.51]
2018-Jul-29 10:00  [-2.78]  1.39840   0.38522   172.94
2018-Jul-31 08:00   -2.77   1.39714  [0.38497]  171.73
2018-Aug-02 06:00   -2.75   1.39592   0.38523   170.11

2018-Sep-14 12:00   -1.74   1.38147   0.50640   128.93
2018-Sep-16 12:00   -1.69  [1.38144]  0.51602   127.49
2018-Sep-18 12:00   -1.64   1.38147   0.52588   126.08


Mars is at opposition on Jul 27, closest to Earth on Jul 31, and at perihelion on Sep 16.
Mars is at peak brightness for several days between opposition and closest approach to Earth.
A difference of 0.01 magnitude is imperceptible to the eye and smaller than the uncertainty of most ground-based CCD photometry.
Albedo features and dust storms probably contribute at least that much variation in magnitude anyway.