Arbok's offensive stats are 85 attack and 65 sp att, and Serperior's are both 75. Using their boosting sets and attacking Mew because it takes neutral damage and has equal defenses:
+1 252 SpA Serperior Giga Drain vs. 0 HP / 0 SpD Mew: 127-151 (37.24 - 44.28%)
+1 252 Atk Arbok Gunk Shot vs. 0 HP / 0 Def Mew: 220-261 (64.51 - 76.53%)
Arbok simply deals more damage.
Serperior has better special defense and resists water (which is unfortunately seen with ice attacks), while Arbok has better physical defense with Intimidate and resists fighting (which is unfortunately seen with ground attacks).
The major advantage Serperior has is its speed, and this is a significant advantage that should always be kept in mind.
As far as attacks, Arbok has Coil+Gunk Shot+Earthquake+Aqua Tail, while Serperior has the somewhat lesser Calm Mind + Giga Drain + Hidden Power. But in exchange it can also use a completely different moveset that relies on Substitute+Leech Seed, or set up Light Screen and Reflect.
There are some other options that probably aren't very useful, such as a mediocre Coil Dragon Tail defensive set common to both, a Leaf Blade Coil Serperior, a Shed Skin Rest Arbok, and some moves of varying usefulness on Arbok such as Crunch, Seed Bomb, and Fire Fang that can be used for specific threatening pokemon.
Overall, Arbok has more power and better attacks, but Serperior has more speed and therefore a better chance to actually be able to use its attacks and can also use supprting moves.
But stats are only a small part of pokemon! There are other reasons to choose between Arbok and Serperior that may have nothing to do with numbers like 85 and 75.
Maybe you like Serperior better because its a starter pokemon and you get to bond with it through the entirety of your pokemon journey. Maybe you like Arbok better because you grew attached to it from older games when it was the snake of choice.
Maybe you just want something whose defining feature is a large demon face! I don't really know, but let the world know by replying to this thread with your opinions!