There are several things to consider:
Antonica has Gnolls, but you'll outgrow that zone at level 20.
Splitpaw Saga (adventure pack) has Gnolls, and is available levels 20-50.
Zek has Orcs, they are tough and you'll spend much time there, around level 30.
Sinking Sands in Desert of Flames (expansion pack) has Orcs, around level 48.
Sinking Sands also has Gnolls in a huge dungeon, Living Tombs, around level 52.
There is no one that will be the best, both are very useful. If I were to pick again, I'd go with Gnolls since Splitpaw has content available for much longer.
But then if you don't get that adventure pack, that arguement goes out the window... :)