Is spring or summer better for a Whitney Mountaineer's Route permit trip?

It depends on which set of skills you brought. May and June offer consolidated snow in the couloir, which means crampons, ice axe, and a genuine snow climb. Counterintuitively, spring can be the safer window on this route, because the snowpack consolidates the notoriously loose rock and gives you clean, predictable climbing instead of rubble.

July through September melts the couloir out into loose Class 3 scrambling. No snow hardware needed, but rockfall becomes the dominant hazard, rated high on this route in late season, and every party above you is a potential source of it. A helmet is worth carrying regardless of month, and doubly so once the rock is bare.

Either season, the constants: about 12 miles with 6,100 feet of gain, altitude sickness as a common trip-ender given how fast you gain elevation, and route-finding on an unmaintained path. Water comes from the North Fork creek on the approach, Iceberg Lake, and snowmelt in season. Most parties improve their odds by camping at Upper Boy Scout Lake at 11,300 feet or Iceberg Lake at 12,600 feet, with at least one acclimatization night built in. Overall success runs about 75 percent.

A fair rule of thumb: comfortable on steep snow, go May or June. Confident scrambler who hates crampons, go late summer and start early to beat both rockfall traffic and afternoon storms.