Shorty packs are lighter and smaller. Most 4WDs are not yet designed to accept the batteries, but it fits in every current 2WD as far as I know.
For 2WDs it means you can have a lighter car if needed - if not, you can place the weight anywhere you like - and on many cars you can move the shorty pack further forward or back to tune the weight balance with the battery itself (instead of using extra weights). Lastly, the space you save can often be used for other electronics, placing the weight closer to the centerline of the car (reducing roll - pretty nice on high bite tracks if you ask me).
I moved to shorties when I was racing for only a few months and never looked back. It works very well for me!
