Hi there,
I’ve got everything for testing my games/apps (Mac, PC, iPhone, iPad) but no good Android-Device (still using a Nexus S ^^).
Is there a device which would be the “best” for testing with Unity?
Maybe the Samsung Galaxy S5? Or something cheaper? ;D
Maybe something with the latest Stock Android 
Go with an older device, you want to see if the game runs well on non-flagship devices. I think Galaxy S3 (slower, 720p screen) and S5 (Still considered a fast device and 1080p screen) or equivalent are a minimum for testing. You need more then one device because from my experience higher res screens act differently when it comes to touch gestures.
Emulator maybe better for indie developers.
An emulator is next to useless for testing on.
For mobile I’d pick something that represents the lowest spec hardware you expect your game to run on and then test on that. If it works on the lowest spec hardware then you can be reasonably confident it will work well on better spec hardware. This does not always hold true though since a lower resolution lower spec’d device may work better in some cases than higher resolution higher spec’d devices (due to limitations sitting in CPU rather than GPU, etc).
Anyway… Yeah… That…
Ok, so a Galaxy S3 and S5 would be a good start for testing?
Should I buy the old S3 (i9300) or the new “S3 neo” (i9301)?