The Longbow: The longbow apparently originated in Wales and found widespread favour with the English during the 12th through 15th centuries. In 1242 Henry II decreed that that all with annual incomes of 2 to 5 pounds should be armed with longbows. The longbow was most often made from yew, but also witch-hazel, ash or elm could be used. The longbow measured about six feet in length and had a pull of between 80 and 150 lbs. Arrows were made of ash or birch, were three feet long and had goose-feather flights. The maximum range was about 400 yards, but actual "killing" range was probably only half of that. Rate of fire was about ten to twelve arrows per minute and at close range (perhaps out to 50 yards) the arrows penetrated any armour of the time. In England continual practice with the longbow on Sundays and holidays was compulsory. This made the English longbowmen deadly. Also note that very few other personal missile weapons could measure up to the performance of the longbow until the 19th century.

The Crossbow: Crossbows are not allowed in Bretonnian armies unless you use the Unofficial Bretonnian Army Book.

Historically the crossbow was probably invented in Han dynasty China and brought to Europe at the time of the Roman Empire. The Romans used the large ballista version of the crossbow as a war machine. Hand held crossbows seem to appear in Europe in the 8th century. They were widespread by the 12th century (contrary to popular belief, crossbows were used on a fair scale in England also). As a military weapon the crossbow declined by the end of the 15th century, but was used later in some countries. Early crossbows had wooden bows and were never as powerful as the longbow. Later constructions used composite materials and had to have some kind of spanning aid. By the 15th century the crossbow bows were made of steel and had to use even more devious devices for spanning. The bow was about three feet in length and fired one to one and a half feet long bolts with leather, wood or metal flights. Most composite or metal bow crossbows had a range of 400 yards, could be fired on a flat trajectory or as falling fire to penetrate helmets and shoulder armour. At short range the bolts were unstoppable. Rate of fire was two to three bolts per minute.

Longbows and crossbows were about equally common during the 12th through 15th centuries, but used more or less frequent in different countries.

The construction of a crossbow might look complex, but I've actually seen one made (complete with trigger mechanism) from fallen trees in a few hours, using just an ordinary hunting knife as a tool. I myself made a bow-less crossbow with metal mechanism using modern tools in about two days.

By these facts we can draw the conclusion that longbows and crossbows had about equal range and penetrataion. They could both be used from front and rear ranks. The crossbowman needed less training to be accurate with his weapon, but the longbowman could loose five arrows in the same time span as the crossbowman shot one.

This means that these two weapons are slightly misinterpreted in Warhammer terms and should probably have been given different game mechanics to account for their historical uses.