What are all the differences between polymorph and death ray?

I am thinking that death ray seems to be more convenient.

Sometimes sheeps pass through.

However, I noticed that husks and skeletons are immune to polymorphs. That means polymorphs "snip" high value target.

Does that work the same way with Death ray?

Basically, please correct me if I am wrong.

So differences are, as far as I know,

1. Death ray insta kill. Polymorph turn enemies into sheep
2. Polymorph select the target
3. Polymorph is 50 gold cheaper
4. The towers are different

Is that really all?