The Uchigatana is available at character creation (Samurai class). The Nagakiba comes from Yura's quest shortly after. Both are regular katanas that accept Ashes of War, but they differ in reach and speed.
Uchigatana
Fastest katana gameplay
Strengths
- Available at character creation (Samurai)
- Slightly faster than Nagakiba
- Lower weight for easier equip load
- Second copy from Deathtouched Catacombs for dual wield
- Classic katana feel
Weaknesses
- Shorter reach than Nagakiba
- Less damage per hit
- Running R2 has less range advantage
VS
Nagakiba
Maximum katana range
Strengths
- Longest reach of any katana
- Running R2 poke is incredibly safe
- Higher base damage than Uchigatana
- Same Ash of War flexibility
- Range advantage in PvP is significant
Weaknesses
- Requires Yura's questline
- Slightly slower than Uchigatana
- Heavier weight
- Only one per playthrough
Verdict
If you value range, Nagakiba is strictly better. If you want to dual-wield katanas, Uchigatana is easier to get two copies of. For single-weapon use, Nagakiba's range advantage makes it the superior choice.