We all feel you and it’d be the same burning question if you;d come from Blaze too 
The good news is that either choice is a pretty good choice.
Theres some great advice from serkandurusoy in the Mastermind group thread: Meteor Mentoring Group aka Meteor University - #48 by aadams
How to start a new meteor app, what to use?
Don’t get caught up in the distractions, there will be a tech of the day every coming year
Follow the official meteor guide, the topics and patterns are relevant to all those stacks
Choose the right tools for the right job. Atmosphere has great forms and tables packages, so a line of business app can benefit from blaze more than it can benefit from react where a game-like interface packed full of “components” may do better off with react
mongodb is not a bad database. database programming existed 50 years ago where there was no such thing as acid and developers had to implement those paradigms within the app itself, use those ideas on top of mongodb
My pennies worth: React is the way to go if your app is a bigger/longer-term project (beyond 2016) and you need to be forward looking on the data/back end.
Perhaps crucially => first class npm support and React ecosystem should be factored in against benefits of current atmosphere packages.(No question MDG have made the right call - for now, at least 
)