What are the advantages and disadvantages between using KEGG or Reactome? As enrichment analysis a usual step is to infer the pathways enriched in a list of genes. However I can't find a discussion about which database is better. Two of the most popular (in my particular environment) are Reactome and KEGG (Maybe because there are tools using them in Bioconductor).
KEGG requires a subscription for ftp access, and for my research I would need to download huge amounts of KGML files I am now leaning towards Reactome

Which is the one with more genes associated to pathways ? 
Which is more completely annotated ?
Is there any paper comparing them ?
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 ...