The Li-ion cell typically used in laptops has capacity of 2000mAh – 2400mAh. 6-cell packs are most common. ThinkPad X-series might use a 4-cell pack due to the form factor. If you are on the move a lot, go with 8-cell. It will add about half an inch.
The common cylindrical cell used in laptops weighs about 50 grams. So 4 cells will add about 200 grams. I am not sure how they configured the pack, but it seems four cells are connected in serial to make 3.6×4=14.4V. The 8-cell pack must contain two of these in parallel to double the capacity. If this is how they designed, there will be no 6-cell battery available.
