Giving a game a few demos before starting is a great idea. Even the best games on the market won't suit everyone. The quickstart rules for both warmachine and hordes are on the
PP site free to download. It could be beneficial to look them over before you dive in blind.
http://privateerpress.com/warmachine/the-game
http://privateerpress.com/hordes/the-game
There is a great starter thread right here on Dakka with tons of resources for new players
http://www.dakkadakka.com/dakkaforum/posts/list/272107.page
I also personally started a thread on a different website about choosing your faction and tried to make it comprehensive. This led to it being kind of long though so copying the whole thing over to a response isn't ideal. I hope a link will be enough.
http://www.beastsofwar.com/groups/warmachine-general-discussion/forum/topic/choosing-your-faction-in-warmachine-and-hordes/
Coming from a
GW background you will probably find some of the concepts quite alien at first and it can be hard coming to grips on the differences. List building is more free-form with more viable options but there is a much greater focus on tactics over strategy once the game has started. There are really too many differences to go over and it would be much easier for us to respond to specific questions.