What are the advantages and disadvantages between using KEGG or Reactome?
One big downside of KEGG is the licensing issue. One big advantage of Reacome are various crosslinks to other databases and data.

ad 1, This depends on which pathway, they are both primary databases. Sometimes other databases that for instance combine data of primary databases have better annotation of pathways (there is an example in the review paper bellow)

ad 3, There is very extensive relatively new (2015) review on this topic focused on human pathways: Comparison of human cell signaling pathway databases—evolution, drawbacks and challenges. However I could not find there which one is more complete ...