You can get all the dirty details of the battles points, experiences etc.. in
the game rules, there: http://www.urban-rivals.com/gamerules.php?
Which is stronger a 4 star card with two power attack or a 2 star card with two power attack?
Or are they the same?
The thing that matters are the stats of your card.
I've seen some bonus that say Power # and some that say Attack #.
What is the difference between power and attack?
Your characters have 2 basic stats, Power and Damage. Attack is equal to power * Pillz and represent the amount of chance your character has to beat its opponent. If you have a character with power =3 and you spend 3 pillz on it, its attack will be 9 (3*3) if its opponent has an attack of 5, your guy has 9 chances out of 14 (9 5) to win vs. 5 out of 14 for the other one. Some bonus increase your power of reduces the opponent ones, and other modify attack. Both are nice and allow different things. Power bonus are cool but you need to use pillz, attack bonus allow you to have decent chances to win wihtout having to spend much pillz. I Hope you find this usefull
ENjoy the game