Shinobu and Rukia are both characters who really feel no mercy towards their opponents. They do their job and keep it moving. Shinobu uses poisons to amp up her attacks while Rukia relies on ice. Their methods are different but they both rely on quick fighting styles. The reason why Rukia would have the edge here is because she’s a lot faster. With her Bankai she can also make it very hard for Shinobu to approach through the sub zero temperatures. I don’t think any breathing techniques would be enough to compensate for that either. Rukia wins.