VS 
first, note the that since this is in game, it comes down to very little, and I would use my fav Pokemon in this case. but since you are asking, lets take a quick look at each of them
venusaur
pros:
- two useful STABS in sludge bomb and giga drain
- a good staller due to sleep powder, leech seed and synthesis
- great special attack
- absorbs toxic spikes and grass sleep inducing moves
- can mega evolve, meaning it looses its major weakness to ice and fire
- chlorophyll
cons:
- chlorophyll isn't really useful in game
- stalling takes to long time in game
sceptile
pros:
- very good speed
- its best STAB: leaf storm combined with its good speed means that it
can usually OHKO and switch out if its levels is a bit higher than
the opponent
cons:
- rather pathetic typing, being weak to 5 types
- shabby movepool
- very frail
- weak to all status
well, sleep powder and stalling is extremely useful competitive, but not particularly useful in-game, where raw power is much more important. sceptile is incredibly fast and can use Frenzy Plant and switch out assuming your opponent gets OHKO'ed, so I would use sceptile in game. competitive: venusaur all the way
