The thing is, crossbows vary a lot in power. In general they are far and away, loads more powerful than bows. Old english longbows can top 160lbs in draw weight easily. Some authentic samples that were tested to destruction topped out at around 175. The problem is you have to be ridiculously strong to use a bow like that, and even then it causes your skeleton to deform and these bony spurs start to form because your body is trying to support such strong weights.
Most modern hunting crossbows top out at 150lbs draw weight, they use advanced compound pulley systems to achieve that instead of raw spring power. Despite the strength of the xbow limbs most are cocked by pulling a lever or other such device.
In ancient times most crossbows were used for hunting. They had varying weights and that was reflected in the method used to cock the string. Most, if not all of them had a loop on the front of the crossbow. You stand in the loop and pull the string back by hand. Another method was the same thing only instead of by hand, you use a belt mounted hook to pull the string back. When sieges started happening regularily, they started making much more powerful crossbows. These were impossible to cock by hand and most used some sort of pulley or ratchet system.
Generally reffered to as "arbalests" siege crossbows made with steel bows had draw weights in the area of 5000lbs! Those crossbows would be able to pierce any armour of the day, and even some fortifications.
So the question is, what kind do we have in DF? Imo, the selection should be expanded to include all types. The method and speed you can re-cock the bowstring should also be very important.