Run time is going to be dependent on how hard the throttle is being used. You can safely assume a 2s or 7 cell pack of ~5000mah will get you anywhere from 10 to 25 minutes of run time.

LiPo's generally require additional steps to care for them. They don't like to be left fully charged or discharged, they don't like to be over-discharged, and you need to use a balance charger to charge them. They have a risk of fire hazard as well, though this is typically avoided entirely with proper usage techniques. However, they can be recharged quicker, have more consistent power output, and can offer longer run times than a nickel cell counterpart. I'll let others comment on the quality of Traxxas and Venom packs, I have no experience with them.

I can, however, recommend you a good charger that will last you a long time. I would suggest the Hitec X2 Black Edition, it has 2 charging ports to let you charge 2 packs at the same time, can be used at home or out at the track, and is fully programmable for any hobby chemistry type out there.