I would definitely go with plywood. Particle board or MDF is useless if it gets wet. I remodeled my kitchen last year, and though I bought my cabinets instead of building (simply didn't have the time) my biggest requirement was solid wood. Which, for cabinet makers apparently, plywood is solid. Plywood will last longer in my opinion, and there's no real need to make the box out of anything real nice since it's never seen. The end cabinets you can always just face it with veneer.
I can see how some of the pressed engineered board would be more dimensionally stable. Still, I feel plywood is a higher quality option. 3/4" sounds thick to me, but you probably wouldn't need any additional bracing in the frame to mount them on the walls.