I would plan it, but be flexible. IMO, the Grand Canyon is not to be missed, so unless it was really snowy and roads were closed, I'd make every effort to get there. Bring plenty of layers, hats, gloves, scarves, etc.
I also agree that Zion is really awesome, but truly, nothing in the world can compare to the Grand Canyon. FWIW, when I was at Zion in March of this year, it *should* have been warm, but there was still snow on the ground in many places, and it was cold and drizzly and foggy. And I've been to the GC in late fall when temps were moderate and it was sunny. So really, ya just never know.
mommy to DS who is 9
DD who is 6
and my girl in heaven