Maybe this even empowers the EoTG then? "If the Engine of the Gods rolls a 14-17 on its table and you cannot summon any unit, then you must instead choose any of the lower results on that table."
It really depends on the rest of your army. Don't take it because of what it alone can do, take it based on how it synergises with the rest of your army. The Engine can be the most broken monster ever made but if the regular Stegadon synergises with your army as a whole better then you take the Stegadon because synergy is the main part of AoS.
Everything does depend on something! Sunfire is awesome against hordes, and the Skystreak bow seems more useful in more situations (to me). I haven't gotten a Stegadon for my collection yet, but I plan to soon enough, and it's definitely getting a Skystreak bow on it. The main reason I'm getting the stegadon is for the Skink Alpha so it can act as the defacto leader of my skinks to replace the Skink Chief in the equation (I always like making skinks strong, and incorporating that into an offensive army? that's the direction I'm going to take my army soon). The Skystreak bow part is because I want to shoot down what ever he has hiding behind the lines, that way I can hit their wizards, or anything else they thought they were "protecting", and with -1 rend helping out on a character they probably didn't think to cast mystic shield on. That's my reasoning though, and I might play a few games and find out it's a dumb idea, haha.
For me it's a case of "the Skystreak Bow will be useful more often than the Sunfire Throwers, but I have other things that can do the Bow's job better and nothing that can really do the Sunfire's job." That's why I went for the Sunfire Throwers, for the army that I had it granted me much higher levels of adaptabilty, which I maintain is the best trait an army can have. But the key words there are "the army that I had." The decision on which armament to get your steggie should depend on the strengths and weaknesses that the rest of your army has and which role you need filling more. And to be frank, even if you get the choice wrong you still have an absolute monster in melee.