Not an easy choice! The Dreadquake combines the Deathshrieker’s large template from the Incendiary rockets with the strength/wounds from the demolition rockets, that’s def a plus. It’s also tougher and that anti-swiftstride mechanic is it’s unique selling point. Against armies like Bretonnia that is a very strong case for the Dreadquake. The Slow Reload is of course a major drawback, as you’re probably only have a few rounds of shooting to begin with. And it costs 50% more, so if you have enough points, 2 Deathshriekers probably give you better output.
The math for scatter/artillery is not easy, but statistically 1 of the 3 Bolt Throwers will hit each turn. That’s def better than a Deathshrieker! Then again the Deathshrieker can fire indirectly, it’s also tougher and can causes panic tests, plus of course the chance to deal D6 damage, so a lot of consideration. A big difference obviously is how many model can you actually hit. When you play some MSU army or linehammer zaelot, a Bolt Thrower will only hit one model, whereas a direct hit with the 5" blast template form the Deathshrieker will maybe hit the whole unit. Check this thread for a more detailed analysis on how effective templates can be. Still, I think point for point a 45pts Bolt Thrower is the better deal than a Deathshrieker.