I have done comparisons between iClone and Carrara and the rendering in Carrara is better, especially if you follow some of the proposed tweaks such as setting the gamma. In general, Carrara's renderer is more tweakable. There is also a plugin to use LuxRender, but I found that so slow that it is unusable for anything other than stills.
You can use DAZ assets in iClone, but with many characters in the scene the project would be difficult to manage, response would be slow, etc. I would say that a total of 500,000 faces would be sort of a max for the total project. Another problem is that textures for a DAZ character are divided over many maps, which also make things slow. You could use Texture Atlas in DAZ to help with that.
iClone is a great tool for animation and more versatile than DAZ, so that would be a benefit. And once DAZ characters have been imported they can be converted to non-standard characters and iClone motions can be applied to them. You can even apply facial animation (at least for DAZ Genesis 1), but I've found that that doesn't import into Carrara.
Any polygon reduction would have to be done in DAZ using Decimator; 3DXchange does not have that option.
You could imagine a pipeline were you import DAZ characters and assets via 3DXchange into iClone, then animate in iClone, then export as FBX (or OBJ for static props) for use in Carrara, where you can do your rendering.
One thing to be aware of is that, except for basic content, most iClone assets, including motions, require an additional export license. So you can apply the motions in iClone and do rendering there, but you would need an export license for importing the characters with motions into Carrara.