Sothe IV

I say speed. 38 spd after Life and Death is not enough to be relevant in the current meta. 41 speed is a lot better imo

Spd

+SPD is ideal for builds with his default Peshkatz, but +ATK is better if you are planning on building a one-shot build using Poison Dagger or Kitty Paddle for him.

Both will work very well, but I would choose +atk, since that puts him at 58 atk with life and death, which is enough to OHKO a pretty much units. He needs +speed less, since Peshkatz boosts his own stats by 4 four after combat, and lowers the stats of the opponent and every enemy within two spaces by 4. This effectively gives him +8 speed, which is enough to double almost anything. (I've almost never seen any unit he couldn't double after peshaktz's effect) -hp also lowers both his magical and physical bulk, so he can't safely take some damage to get into desperation range (or any other skill's). I personally use dragon fang as a special, with heavy blade in his seal slot, and +atk makes that special stronger too.