The sisters are a pretty difficult army to play. They have little in the way of armour, or anti armour. Their HTH effectiveness is dependant upon their characters, faith points, and the abbility to take a significant chunk out of the enemy before they get into hth, because of their low point cost and good survivability they can lay down a lot of fire with bolters, heavy bolter, and flamers.
They are an expensive army to purchase because of a complete lack of plastics.
The sisters are a fairly flexable army that doesn't have a lot of variety.
If you want an army that is the complete oppisite of tau then the tyranids are the army of choice, because of the nids reliance on hth combat rather than the tau who utilize shooting.
If you would like an army with more options but retaining tactical flexability then Eldar, Codex marines, Chaos Marines, or even necrons would be a better choice then the sisters, especially if you haven't been playing for long time.
I chose Chaos for my 2nd army. The variety of units and the background was what attracted me. No one has more units availible to them then a chaos general.