Traxxas packs are more expensive because of their propriety "ID" function. To use this you also have to use their charger. It's basically a dumbed-down system where the charger "recognizes" what type of battery was connected and automatically sets most of the chargers settings for you.
For NiMh packs you can use the Traxxas packs with any brand of charger but you simply have to set the charger up just like with any other brand battery. You can also use the Traxxas charger with any other brand of battery, you simply have to set the charger up manually.
For LiPo packs you will need an adapter to use the Traxxas packs with any other brand of charger because the Traxxas packs have an internal cell balancer. With this adapter you connect it up just like a typical LiPo pack and set-up the charger just like with any other LiPo pack.
You can use any brand of NiMh pack or LiPo pack with your Slash. I would recommend using batteries from Venom, Dynamite, Reaction. If using MiMh packs get a 7-cell pack with a high capacity for longer run times ...say 4500 to 5000mah rating. If using a LiPo pack get a 2-cell pack again with a higher capacity such as 4500-5000mah.