Grand Canyon Visit
 

Just wondering the best time to visit the Grand Canyon and/ or Zion National Park?

I would pick May or September.

Would the heat be unbearable in July?

No. High elevation around 7000' and above at the rim.

The crowds and lack of available lodging will be a problem in July. If you hike down to the bottom, the heat will be unbearable, but the rim area should be bearable.

October

April.
Could be cool, but hiking below the rim will not be unbearable. Crowds will be smaller than during the summer. Hours of daylight will be much higher than during the fall months.

I think everyone except Starrs is ignoring the Zion part of the OPs question. Average high is 100 in the main canyon, that combined with the crowds would be fairly miserable for me.

Fall or late spring is ideal for both parks. Fall is a little better because there shouldn't be snow/ice lingering on the trails and the Narrows won't be running so high with snowmelt.