It's really hard to tell until about 10 years down the road. I think it depends as much on the quality of wood as which pressure treatment is used. I've had boards from the same batch last 20 years and rot after about 10. I have had good luck with the weather Shield at Home Depot. It may be just a little bit more expensive than what you might get at Menards, but I would say it is probably worth it.
These a pine with a cedar tone pressure treatment. They look like cedar but are smooth cut. Wait a year and you can stain them darker cedar tone color. And they are about half the cost
They recommend waiting a year. The treatment needs to dry through first. Otherwise your stain will be blotchy
12’ is the longest I saw in store on these. I know the 5/4 x 6 cedar tone boards are available in 16’ in store. 16’ seems to be the longest they carry in stores not sure if you could special order.
2x4 is actually 1-3/4”x 3-3/4” 2x6 is actually 1-3/4” x 5-1/2”