This is a complicated question, but I will tell you what I have learned (and other people may say I am dead wrong!)
I would say that PhD programs in I/O psychology are easier to get into. For one thing, I think there are fewer applicants and therefore less competition, but I have also seen schools where they take a proportionally higher number of students into the I/O program. However, these programs weed out a lot of students with comps/quals. At my undergrad university, about half of the I/O grad students did not pass their comps and ended up leaving with a masters. With a masters in I/O psychology you can still get an awesome job, especially if you are looking for something outside of academia.
I haven't seen any social psychology programs where many students leave with just a masters, most of them tend to take fewer students but see them all through to getting their PhDs.
If you are dead set on getting a PhD, you can probably succeed in either area. If your interests are between social and I/O, it's probably a good idea to apply to programs on both sides.