Spirit is better because she has a higher base speed than talia by a wide Margin. You can mod her fast and to hit hard and she can really disrupt a few of the NS counters.
Talia seems good on paper, but she has very Limited use in the team. NS teams with talia are very easy to beat, especially with imperial troopers. A fast Spirit opens up the battle and can stun someone quickly, while also adding a stack of plague if it is MT lead or reducing their Tm if it is Asajj lead.
She adds more stuns to the stun train of Daka and ventress, hits quite hard, is very fast (the fastest base speed of the Nightsisters which is vital to stop counters), and a relatively easy character to G12!
I hope this helps!