Beams are probably cheaper in the long run. Considering how beams are equipped on almost every capital ship, it is safe to assume they don't require a constant supply of "rare gases". The logistical forces will not be able to supply bombs rapidly enough where beams require little or even no additional support. The Mjonior (gah, can't spell that) cannons are relatively small and show no signs of needing a large amount of gas or other supply. It can be safe to assume that beams probably do not require much or any support where bombs can pose a problem if the supply line is broken. Remember, capital ships are meant to be able to survive behind enemy lines. Fighters and bombers on the other hand, must rely on support and capital ships to return to.