This is rather a long comment... (This was originally written for http://meta.stats.stackexchange.com/q/2253/25741, but then I found this question).
Comparison by tags
One way to look at datascience.SE (DS) / CV is to compare the sites by tags.
- machine-learning: 681 (vs. machine-learning: 4707 on CV)
- data-mining: 246 (vs. data-mining: 902 on CV)
- r: 246 (vs r: 11973 on CV)
- classification: 231 (vs classification: 2329 on CV)
- python: 218 (vs. python: 749 on CV)
- neuralnetwork: 152 (vs. neural-networks: 1242 on CV)
- bigdata: 151 (not on CV?)
- clustering: 134 (vs clustering: 1717 on CV)
- dataset: 130 (vs dataset: 798 on CV)
- predictive-modeling: 116 (vs predictive-models: 953 on CV)
- r: 11973 (also on DS)
- regression: 9419 (also on DS)
- machine-learning: 4707 (also on DS)
- time-series: 4631 (also on DS)
- probability: 3564
- hypothesis-testing: 3290
- self-study: 3214
- distributions: 3022
- logistic: 2860
- correlation: 2543 (also on DS)
One can see a couple of things here, I think:
- CV is much bigger than DS, by now. Considering the fact that DS is only 623 days in beta (source), but CV is 5 years and 6 months old (source), I guess this is natural. It would be interesting to get some data for stack exchange sites to try to predict the growth :-)
- DS seems to attract more people from computer science, where CV seems to attract more people from mathematics.
Further analysis
It would be interesting to get a graph of the tag growth by month for both sites (in one graphic).
I would also like to see which kind of questions get closed on both sites. Which tags do they have? How often does it happen that a question gets migrated DS -> CV and how often CV -> DS?
Place of discussion
I wonder why this is discussed here. It seems to me that you have to get very biased answers here, whereas you could also ask it on http://meta.stackexchange.com/.
Discriminating stats.SE from datascience.SE
For me, I can say that I like DS more. The name of the site seems to be more clear to me. Only from the name, I know that this includes machine learning / analysing data / classification / prediction. But cross validated? I know what cross validation is, so is CV only about testing? And why is it called "crossvalidated" but has stats.SE as an URL? This seems unfortunate.
When one likes to have one big site, then why not merge both in math.SE / stackoverflow / cs.SE / opendata...?
I think both sites have a reason to be there. It seems to me that stats.SE should be about statistics. Yes, it has a large number of machine learning questions, but my guess is that this is simply because of the age. StackOverflow also has a lot of ... well, everything. Because it was there first. That doesn't mean the other sites are useless or that new questions shouldn't be moved. One example is the latex tag on SO. Almost all questions tagged with "latex" on SO get moved to tex.SE. Similarly, I think almost all questions tagged with "machine-learning" should get moved to datascience whereas "statistics" is a candidate I would rather see on stats.SE.