Due to Dutch Elm, Elm needs to be debarked if stored for firewood.
And I concur. It sucks to split.
Citrus and Avacado are less dense woods and burn rapidly. A cord of each is cheaper than Oak but you have to use twice as much of the others to keep a nice fireplace fire going. Oak is great.
For more information than you needed…
http://www.chimneysweeponline.com/howood.htm
Stay away from pine for a fireplace because it pops way too much, sending sparks out into the room (even through a screen). Also don’t be tempted as I was to use some kiln dried lumber scraps (1x6 poplar) for kindling – same probem.