You get XP like you get Hero XP: with actions in game.
10 XP if you win
1 XP is one played card/ used hero power
0,5 XP is attack, trigger, etc
So right now it does not matter if you play 5 aggro games or one control game you get the same XP. I would suggest spell mage because you generate cards very fast and have a lot to play. The new system rewards time and not wins so much. Even when you win instantly you get just 10 XP for the win so fast matches are not worth much. (cap at 400XP per hour because of bots)
If you are a control player you will have a great time... as an aggro player you could get annoyed.
I have to say that I dislike the new system just because how hard it is to get XP later... I would like to have 2500XP for every level and turn down the rewards at the end.