I notice that the current version of Marlin has delta kinematics, does that mean that the various delta-oriented branches have been merged in now?

Which is the best fork/branch to use these days?

The only differences I have found are that Erik's Marlin firmware [github.com] uses bed leveling in a square pattern, and Johann's [github.com] uses bed leveling in a more circular pattern (or so I've heard and seen on youtube).

Or even better just head over to Richrap's blog and read his firmware setup guide for the 3DR: [richrap.blogspot.co.uk]