For me, I have both plastic folding sawhorses that I like to take to jobs. I have an 8 foot bed on my truck, and space is always limited, so being able to fold and allow me to stack other stuff on them is more important then size.
The main reason that I use saw horses is for ripping plywood when I don't want to get my table saw out, or bring it to a job. the only other time that I use saw horses is to create a work table or platform. I recently refinished a 90 year old table and I set my extension ladder on the saw horses, and then clamped a 2x8 to the ladder with some screws so I could stand the legs of the table into each screw. I also put the table on the ladder so it was at an easy to work height.
When ripping plywood, I set my blade so it's just below the thickness of the plywood and then I cut right through the top of the saw horse. My folding plastic saw horses last a couple of years, then I get new ones. I also have wood saw horses that I built from scrap that are 3 feet wide and about 3 feet tall. They are more heavy duty and take up more space in my truck, so they usually only get used at home.
For cutting lumber, my chop saw is my first choice, but on a lot of jobs, I don't want to haul it around with me. Especially if I'm cutting just a few 2x's. Chop saw is for trim work, or bigger projects when I'm using it a lot. For that, I put it on my tailgate and use rollers on either side to support the material. The rollers extend up and down so it doesn't matter where I'm parked, I can quickly adjust them to the height of my chop saw, or the height of my tailgate if I'm using my cordless saw. I use my rollers a lot more then I use my saw horses.
