Rather than a comment discussion above I'll highlight what I see as points to consider.
Consider only phones (for sake of discussion). If you want to buy ten devices, I'd buy just one iPhone, one or maybe two Windows phones (if you want to support older phones), and seven or eight Android devices. It may or may not be apparent to you, but
- all iPhones have the same or almost the same hardware specification
- almost all iPhones are running the same (latest) operating system
- the API is well tested against the hardware - what runs on one should run on all
- all Windows Phones have a strongly defined minimum hardware spec, including fixed resolution
- there are only a couple of operating system variations, and the majority of phones are running the latest OS
- the API is again well tested against the hardware, because the hardware is strictly defined.
Compare with Android
- there is no hardware spec, no minimum requirement
- there is no manufacturer licencing, so there are dozens of device makers using Android (cf one for iPhone, and just a handful for Windows)
- there is well known operating system fragmentation; 2.3 is the most common OS in the wild but millions of people have 4.1, 4.0, 3.0, 2.3, 2.2 and so on.
- screen resolution varies wildly - not just in absolute size but also in apsect ratio.
Now if you market is primarily going to be iPhone then get two iOS devices, because there are in truth two screen specs ('normal' and 'retina'). Similarly WP7 has some minor difference between 7.1 and 7.5. But if you want to cover the widest range of devices, spread yourself across Android.