They both have shitty traffic
SLC has Big Cottonwood, Little Cottonwood > Boise w/ Black Cliffs
Boise has excellent whitewater
SLC has better skiiing
Alcohol laws are really weird in SLC, but you can work around them if you live there.
Boise has a better art/music scene (Built to Spill!)
SLC has more 'culture': opera, theatre, etc.
SLC has Mark Twight.
SLC has Grivel, Black Diamond, and a thousand other gear manufacturers, so you're more likely to find gear deals.
Idaho will offer more solitude, wilderness, wolves and grizzlys, Utah will offer crowds, the vacationing elite, and the Beautiful People.
I'm an Idaho guy, and I always dream of going back.