http://www.baseballprospectus.com/ar...articleid=2528
This article discusses 'ratings' for the various winter leagues. Mention of the Mexican League (I'd assume you mean the hitter friendly summer league) is at the very bottom of the article. According to the article, the summer league rates in at .780, which is at or just a tweak under AA level.
As far as the Atlantic League, I've seen an equivalent level somewhere among or between AA and AAA. Of course, the chances of these players making the bigs (being scouted or otherwise) is lower than the official minors. But for Indy leagues, I think generally you wouldn't go wrong with A or AA level. Depends on how many players you want that are skilled enough to jump leagues if called upon.