Oh , if you are looking at the white post for aesthetic reasons, buy a couple gallons of white enamel paint and paint the steel post, it will last much longer than those cheap (cheap made recycled plastic that is) junk that cost double what a good steel T post cost.
We have used some of the plastic post with a steel tip on the end for electric fence wire and they break very easily and I doubt the plastic T post are any stronger, however the main drawback on white plastic is sunlight deterioration. Any plastic that is exposed to UV light needs to have a color UV blocker in it. White plastic can only be used underground or under roof as it becomes extremely brittle is 2-3 years which is what happens to the plastic electric fence post. After a couple years of exposure, they get really brittle and break when even a little bend is put on them.
There is a good reason that you don't see a lot of reviews on plastic post, either square hollow or even the T post, that is because 1) they chalk up and don't last in UV light and 2) they are too expensive.
The theory of no rot is good, but they do rot. Just not in the traditional sense of wood. They rot (oxidize) from UV light just like all other plastic. Bury them in the ground and they last a millennium or more but not above ground.