As a programmer, don't "study" operating systems, study programming paradigms.
But if you're asking whether there's more money in programming for Windows or iOS, it all depends on what you're interested in selling.
If you're into "hip" "apps" and games for teenagers and gadget addicts, Apple's your thing. All those kids are drawn by Apple's marketing and buy every new Apple gadget on the market (and to cater to them, so will you have to do the same).
If you're interested in business applications, lower in volume but more money per sale, Microsoft is probably your thing (but there are businesses using Microsoft desktops and Apple tablets).
Overall, IMO the Surface is a very interesting platform to replace both smaller laptops AND tablets in the business world, and allow near seamless integration with a corporate IT environment based on Windows based desktops and servers, as well as mobile phones.
Apple is trying to create a similar landscape for its products, but they've a lot of catching up to do and even when they get there they have a major hurdle in that most companies are heavily invested in Microsoft products already and aren't going to want to throw all that away just because some junior sysadmin thinks "Apple iz k3w1 and Micr0$0f7 zux".
Each one has it's pros and cons. Windows is more mainstream for PC users, business, and perssonal use. Apple is more known for phones/tablets/mobile devices. Which one you study is up to you and what you're interested in developing for. Follow your passion.