Linear or hierarchical comments to the answers it is better to implement?

The main problem tree reviews — if they become many, it is very difficult to do a sane breakdown of the add on page. And it's good even if the server is powerful and the engine is well-written, and often wait for 10 seconds until the page is formed, and the same, to boot. If they are paged, it becomes difficult to follow the discussion, which is conducted in different branches. Someone solves this problem by pruning branches and uploading thereof on a click, but to put it mildly, not conveniently.

IMHO, hierarchical linear system is much easier because each individual response may be more fully discussed in a separate thread.

I think you need to come more from the reasons many review and how it will look on the user's screen.
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In that case, if the review plan up to a maximum of 10, it is possible to do linear.

another feature local reviews, Q&A — comment on the review not listed under "my review"