Generally blowpipes are better because you generate enough shots to actually cause some hits. The giant bow is notorious for never hitting anything as generally the steggie is moving making it even harder for the barely average BS crew to hit anything. Both howdah weapons are generally on moving steggies so are 5 to hit at short range and 6 at long, bow gets one shot the gatling peashooters get 4D6. With the avarage length of time a steggie survives per game you are lucky to get one hit from a giant bow once every two games, gatling peashootes should get a minimum of 2 poisoned hits per shooting phase. But remember a steggie is for close combat, the shooting is a bonus, and the ancient steggie is generally a better choice if you have the points and room in the rare allowance to afford one.
The shooting should really be a secondary concern for you, the stegadon is, as strewart said, for combat! I love the ancient stegadon with skink chief and stegadon warspear. Ive failed a few overruns/ charges and been in a spot to shoot the next turn. Much more flexibility imo