The entire dairy herd ends up as meat within not much more than 5 years into what, for a natural strain of cows, would be a lifespan of somewhere between 20-25 years.
The dairy herd is actualy a meat herd that instead of only having to endure 2 years before it goes to the bolt gun gets to suffer five years of literaly being over worked to death.
On that basis the question (with apologies for the holocaust anology) is somewhat akin to this:
Which is worse; Two years hanging about in a concentration camp with sweet FA to do and then the gas chamber OR five years painfull hard labour in the concentration camp and then the gas chamber anyway?
Just the slaughter, rather than a few years torture and then the slaughter, is the lesser of two evils so far as I can see.
Given the choices you offer (100 people no longer eating meat or 100 people no longer drinking milk), I would try to convert 50 people to veganism.
The other two don't make that much sense.
On a personal level, I do not find milk as repulsive as meat, and I do experience more personal discomfort at seeing dead animal parts than, say, cheese loaves, as you immediately make the connection that animals need to be killed to produce meat. They are also killed in the long run to make milk and treated quite badly until that, but that is a more intellectual kind of knowledge, if you compare it to seeing a piece of raw meat that was once a living, breathing animal.
Of course, it might also be because I was a vegetarian for 20 years before going vegan.
I was actually thinking about this exact question. Mainly because veganism is quite often a transition that usually doesn't happen overnight. So I wondered if I were to try to convince a meat-eater to at least give up "something", for at least 30 days or whatever, would I have more success convincing them to ditch the dairy or forgo the meat? My gut says they would more likely adhere to a meatless diet than a dairy-free one. On the other hand, avoiding dairy might make the bigger impact in terms of preventing suffering. Is that sort of the spirit in which you asked the question?