I would say if you have all the meta unis and enough to clear SL without problems, then go for Emma .. else maybe better buy some more unis if you are not a big Emma fan.
Emma is great and i really love her, but she is more or less a luxury char just like most pawalls.
I am pretty indifferent to Emma and yes, I do have most of the meta. Looks and sounds like she could be really good but I also want to get to level 70 Maw. My issue is I would be wasting crystals if she goes on sale next all uni sale if I get her but also if I don't and she is never on sale again.
This is my one issue with this game, my OCD and potential buyers remorse makes me overthink things ;).