The latter (assert!(arg3 < arg0, 6);) will be slightly cheaper in terms of gas because it's a single instruction (Cmp) rather than three (Mod, LdConst<0>, Eq). However, it's generally not something to worry about due to gas bucketing. Arithmetic instructions tend to be much cheaper compared to other instructions. Also, the Sui gas model charges differently only for programs with consistently different wall-clock execution times, and arithmetic execution timings can naturally vary quite a bit. More details can be found in the Sui tokenomics documentation: https://docs.sui.io/concepts/tokenomics/gas-in-sui#computation.