6 cell for sure ... for most manufacturers this is maybe a $30 or $40 upgrade at max, and is well worth it. The 4 cell will (1) not last long enough and (2) will die out faster (aka lose charge capacity) because you'll need to charge it up more often. After all, a new battery does cost $100-$150, so I think you should shell out the money for the upgrade.
^About the weird hump: I've heard that it does help cool the notebook if it elevates it at an angle, like HP's 12 cell batteries. But yea the way Lenovo designed it just doesn't look too appealing.