I checked out the HD version in the Google Play Store on my Motorola Xoom tablet. I have discovered that there is virtually no difference really, and definitely not worth the $2.98. Feel free to check it out for yourself, because unlike iOS, Android has the option to refund your purchase within 15 minutes, I believe, of the time of purchase. I refunded mine because the only differences I could find were that the start-up screen after the white "Rovio" screen was not stretched (but every other screen/level I tried was), and also when you are in a level or at the start-up screen where you press "Play", the pause buttons are slightly larger.
I also believe it is pointless to purchase the Premium version on either a tablet or phone, because the only difference is that it is ad-free, to my knowledge. A helpful little tip that Rovio doesn't want anyone to know is that the ads the put in the free version require some form of internet connection to be generated in the game. So to solve this, you could just put your phone/tablet on Airplane Mode if you want since it's simpler, but all you really have to do is disable the WiFi AND data services on your smartphone, and boom - no ads. Obviously, on your tablet (assuming you don't have 3G/4G) all you have to do is disable WiFi. I actually have widgets from HD Widgets (which I highly recommend and is well worth the $1.99) that allows you to disable both data services and WiFi with the tap of a button. And there you go, you've just saved a dollar. Or three.
Hope this helps and clears things up for everyone.