I was too and decided for Solaria, because she seems to be more versatile.
Natalie's voice modes: Soft, Soulful, Steady, Bold, and Warm
Solaria's voice modes: Clear, Soft, Airy, Power, Passionate, Solid, and Light
Hard decision ... I chose Solaria with the SynthV bundle and thought, if I'm unsatisfied, Natalie will be cheaper as a second voice than the other way round
Maybe I'll get her too, but I mainly use male voices and picked Saros as my second voice and NineZero for Rock stuff.
For background vocals Solaria and Mai are a nice pair and if I should need a second female lead vocal, I guess Natalie will be my favorite.
PS: Check Youtube for songs with both ... maybe that helps in your decision.