I have no idea what the policy is if you buy with the free Google Play credits, but regular purchases have a 15 minute window. Meaning, if you go back to the app's Play Store page within 15min, you'll see a refund (or similar) button to click.

Problem is, the publisher of Modern Combat, Gameloft, are f*ckin bastards!! The file you download from Play is basically a small script and template. Once you launch it, it then downloads the game data using Gameloft's own mirror, which is f*ckin slow. Seriously, I'm on 100mbps down/50mbps up speed here (I live in Singapore) and the server gives me something like 4 or 5mpbs speeds, so by the time you finish downloading and test the game (only to realize the controls suck), your 15 minute trial period is long gone.

Seriously, avoid games from Gameloft. They're con-artists!!
Amoleds are more contrasty, but it's often innacurate contrast. You'll never see a professional grade EIZO or NEC monitor use amoled technology over IPS, since Amoled can't be calibrated accurately enough for stuff like print-proofing.

For handsets, it all depends on what kind of Amoled vs what kind of IPS. The pentile Amoled on my Galaxy Nexus still beats the IPS on the Nexus 7 for contrast and overall tonality, but you must realize that the Galaxy Nexus was a totally different price range when released.

On the high end, Sharp's IGZO IPS still beats Samsung's S-amoled for overall accuracy.

Of course there are also a whole list of pros and cons of each, but it's simply too long to list here.