I think each represents such an out of context problem to the other that an abstract comparison is hard to make. Sonja could take Tara in a swordfight or if they were on horseback, but that's not a fight Tara would get into if she could avoid it. I mean really, even someone far less skilled with guns than Tara could take Sonja if they weren't starting within arm's reach.
I think you're right. Given RS's longevity I just assumed she had some decent showings against gun-wielders but she just doesn't. Even if she is way stronger than Tara and has an incredible skill set, RS doesn't actually have a well-documented enough speed advantage, if any, which would be her only saving grace against someone like Tara
The closest thing I could imagine to a fair matchup would be stalking each other through a partially overgrown ruined town, like the Pirpyat/Chernobyl site or Centralia, PA; with Sonja having a sword and some javelins, and Tara a knife and a revolver - with only the six rounds.
I don't know who would be more likely to get the drop on whom, but it could be an awesome ride.