I find programming for iOS easier. I also think the target market on iOS is better quality than the Android one, despite of raw market share they are a lot of small and underpowered Android devices out of there.
If I were creating an startup with a mobile app I would definitely launch on iOS first, test the waters and see how that goes. If it works then I would start thinking about launching in other platforms.
While true, there is little stopping a team from supporting a set of high-powered Android devices.
The difference between Vine and a startup are obvious. Vine has teams for iOS, Android, and specific design, whereas a startup typically starts off with one or two devs at most. Twitter are known for being iOS fans, so they were always going to tailor their product towards iOS.
Vine started out as a small (3 person) startup that Twitter acquired just before their launch. Could they, post acquisition, have gotten the android version out quicker, maybe, but it's unlikely that they could have built the backend, an ios, and an android app with the amount of polish the ios app had at launch and launched at the same time with just the 3 people they had prior to the acquisition.
If I were creating an startup with a mobile app I would definitely launch on iOS first, test the waters and see how that goes. If it works then I would start thinking about launching in other platforms.