One factor when determining choice of platform is the target country. Market research is often global and it can be easy to miss that some platforms are much more popular in some countries. There’s a recent article at cellular news that gives some useful figures for the UK, USA, Germany, Italy, France and Australia.
For example, here in my home country, the UK, Android is now number one followed by BlackBerry and then iOS. However, one piece of advice I often give clients is to survey your current (or potential) users. You may well find that you will need to choose platforms different to those mentioned as top platforms in consumer or analyst research.