What would be the reason one learn to make apps for Windows Phone 7 instead of Android phones? Is there any reason why Windows Phone 7 is the platform that a developer choose instead of Android?
If you already know Silverlight or XNA, and you have an idea for a phone app, you could write it on WP7 in a very short time (developers I know are reporting a single weekend, or a week of evenings) and get it into the hands of users. This will give you both feedback and revenue. Then you can do versions for other platforms from a stable base. If you need time to learn the languages and frameworks you need for other platforms, this is a great plan. Of course, if you know those languages and would need to learn C# or VB, plus Silverlight or XNA, then this is not a good plan.
Also some developers may work for an employer that has decided on a single phone for their staff, and be tasked with writing apps for those staff. The side-load issue prevents that from being a viable WP7 reason right now - apps can only be installed through the marketplace, ruling out internal-use-only apps - but a developer who is confident side-loading is coming might be working on a WP7 internal-use app.
My first question is why is it an "either/or" decision?
If it is (due to a lack of time or resources for example) then the question has to be:
Which platform will yield the greatest return on my investment.
Now I haven't done a complete investigation myself, but you need to look at the cost of the tools, the time it would take you to learn those tools, the cost of hiring someone who knew the tools etc. and weigh those against the potential income you can earn from selling your application on a particular platform. This isn't just the price you can charge for your application but the reach of that platform in your target market.
Only when you have that information can you make an informed decision.
The only reason I can think of is if you already have grounds to develop for Windows Phone 7. If you have no such grounds, don't go Windows Phone 7 route. It was clear back when it was started and it's even more clear now, that WP7 is not a road to success (just 2 mln of devices shipped to shops and operators, no info on how much actually sold).
1) It's virgin terrority. The apps that sold the most for Iphone were the good ones in the early days because it was easier to stand out in a smaller crowd
2) The tools are great. Silverlight and Visual Studio is a great developer experience. Couple that with the .Net framework and you have a really good foundation for developing apps.
3) It's going to be big Ok, this is my personal opinion so please don't downvote me ;) It's my prediction that despite all the negative press and child-diseases Windows Phone is going to be one of the top 3 mobile platforms within 2 years.
Microsoft has the enterprise customers through outlook/sharepoint and office. They have tons of money to burn and they have millions of eager .net developers who want to get into the apps game. Just look at Xbox where MS went from not playing to being a serious contender within a couple of years
The problem right now is that they rushed a product to market and is still playing catch-up. Also there isn't any really crave-able WP7 phone that stands out from it's competitors. When they manage to ramp-up the platform and HTC/Nokia get going with the next generation phones, watch out.