How to protect aluminum fence posts from weed trimmer

A concrete post/form in the ground with some sort of above ground post or embedded post in the concrete is the way to go. Depending on what you want from your fence, ie: privacy, security, looks, etc. will effect what kind of materials to use for the actual visible fence. There are many choices of composite materials and vinyl products out there that are low or no maintenance and will outlast any wooden materials. The first serious structure to fail is usually where a wood post meets the ground. You need to pay special attention to this area, keep wet dirt, leaves etc from building up around the wood posts. The concrete should be sloped away from the post so water drains away and no water stands around your posts. Use good hardware, galvanized at minimum, stainless steel screws are best, but a little pricey.

I'm not sure if either contractor has really given you good advise. Tell us a bit more of what you what your fence to do and look like and we can help you more.

Personal opinion/experience agrees with the advice you are getting. Cedar is a good choice above ground; it's less toxic and as long as it can dry out it resists bugs and weathering pretty well. If the posts will be above ground resting on concrete anchors, cedar is fine. If you are going to bury one end of the posts, though, I would recommend PT. Realistically, the difference in appearance is minimal, especially after staining or weathering. I'd save the cedar for the fence panels.

Of course if you want a natural product that is extremely bug and weather resistant, some of the tropical hardwoods like Ipe are now available for decking and it might be possible to get 5x5 posts. Not cheap, though.