From what I have read, it would seem the Serum has better plug in support, an easier to use GUI, more online tutorials and more presets. So it might seem that when considering which to use, that Serum would be the best choice. However from what I understand Serum has one developer, and another competitor Vital has also just one developer.
From past experiences, when applications have just one developer, then there's a good chance that after some years of time, the products may not be as well supported as those that come from a larger company. Well except Microsoft, who really don't care.
So I feel that given a choice between learning Serum, Massive X or Vital, it might be better to choose Massive X, and learn as much as possible about that synth, which I think is the most likely of the three to have regular updates.
For all those people here, who might have experience of using more than one, can you tell me are there capabilities that are not present in Massive X, that you feel are missing? Also if there are, do you think these capabilities will be added in the future. How about software updates, what's the track record of Native Instruments?