As of conda 4.4, conda activate is the preferred way to activate an environment. Generally, you won't find too much of a difference between conda activate and the old source activate, except that it's meant to be faster, and work the same across different operating systems (the latter difference makes conda activate a huge improvement IMO).

From the docs, regarding the release of conda version 4.4.0 (released december 2017):

conda activate: The logic and mechanisms underlying environment activation have been reworked. With conda 4.4, conda activate and conda deactivate are now the preferred commands for activating and deactivating environments. You’ll find they are much more snappy than the source activate and source deactivate commands from previous conda versions. The conda activate command also has advantages of (1) being universal across all OSes, shells, and platforms, and (2) not having path collisions with scripts from other packages like python virtualenv’s activate script.