Sleep Powder is generally better, since Sleep overall is much more powerful. Grass-types are immune to both Sleep Powder and Leech Seed but Sludge Bomb/Wave takes care of them. Giga Drain/Sludge Bomb/Sleep Powder/Leech Seed is a great set for a tanky (Mega) Venusaur.
3DS FC: 5257 - 9998 - 2632 Pokemon Y Name: Benjamin
personally myself i use a mega saur with giga drain sludge bomb synthesis and sleep powder. this really helps to defeat pokemon venusaur has no real chance of beating as for toxic and leech seed they are effective but it really depends on the opponent saur is more of a tank not a wall so he is definitely weak against moves such as physical flying so its really a personal preferance or what ur looking for as a spot to fill in ur team
FC: 5214-9350-0658 IGN: Dark Friend Safari Magmar, Pyroar, and fletchnider
If it's already level 23, does that mean you've already got the nature, IVs, and EVs you want with it? Are you making it Mega Venusaur or Chlorophyll Venusaur? All of that information would help for making a moveset for this.
If you didn't do any of that sort of breeding to build this pokemon, then really I'd just go with whatever since your odds of using it to any degree of success are extremely slim either way. If you could run sun support with it I'd run Growth and Giga Drain with Leech Seed.