I would probably say Knightwalker. Here's what information I could gather from the RWBY wiki, Fairy Tail wiki, and (of course) the pages from this wiki.
Pyrrha Nikos
Pros:
-Aura allows her to take a certain amount of damage with no injury
-Semblance: Polarity (can redirect Knightwalker's spear)
-Longer attack range
-Extremely accurate [Snagged Jaune's hood onto a tree with Miló (spear form) while he was launched from a cliff dozens of meters away from her position several seconds after she landed from her launch]
-Level-headed strategist (can make quick assessments after a few minutes of fighting)
-Balanced fighter (Adept at all of Miló's forms, can quickly switch between them mid-strike, keep attacking in quick succession, and quickly counterattack after defending)
Cons:
- Physically weaker than Knightwalker
- No means to fight back against several of Knightwalker's Ten Commandment forms other than magnetic redirection
-She must come into contact with Knightwalker's spear for her semblance to take effect
-Slower when Knightwalker has Velocity Spear, Silfarion equipped
-Can't take a hit from Saint Spear, Ravelt (Knightwalker's strongest spear)
Erza Knightwalker
Pros:
-Physically stronger
-Multiple spear forms give her a wide variety of options for attacks
-More aggressive fighter
-Superior speed with Velocity Spear, Silfarion
-Far superior power with Saint Spear, Ravelt
Cons:
-Overconfidant
-Less strategic than Pyrrha
-Shorter attack range
Pyrrha would be able to keep up with Erza at first, landing precise strikes in rapid succession and countering Erza's attacks because of her quick thinking and her semblance. However, once Erza switches to a different spear form, Pyrrha would be forced to defend as best as she could; seeing as she has no other means to counter besides her semblance, that would be very difficult considering that Erza is far more aggressive. Her only option is to stay out of Erza's range and use her rifle to deal damage to critical limbs from afar. Unfortunately Erza would close the gap very quickly with her Velocity Spear, Silfarion. Pyrrha might be able to dodge seeing as she has a high hypersonic reaction and can redirect the spear with her semblance, but eventually she would get hit because of Erza's faster speed. Though she's hit from Erza's spear she won't be injured because of her aura, up to a certain point. However, considering that Pyrrha's durability was never pushed to it's limit before, she may not be able to tank a hit from Erza's Saint Spear, Ravelt. If she get's hit, Pyrrha would be defeated at that exact moment, but it wouldn't matter if it was a direct hit or not because Ravelt would just cause a powerful explosion.
Though this is just my opinion and I may have some errors.