Poly Engine what was good/bad about it and why is the LA better?

Really, it was all about size and manufacturing costs. The LA was lighter due to the newer thin wall castings and the wedge heads made them dimensionally smaller, to fit more easily in cars a like the A bodies (the poly was an "A", "LA"s were "Light A"s if I'm not mistaken...). The poly heads (which were actually quite good for their day) cost more to produce than the LA variety, kind of like the situation with the Hemi vs. wedge BB heads.
Chrysler needed a V8 to put in their small cars, and assembly line installation of a poly into an A just wasn't practical- so they went with the LA and dropped the poly.