I'd take the deathrocket over the bolt throwers (points permitting). As long as you are decent at estimating, you stand a reasonable chance of hitting and killing more models then you could with two bolt throwers.
well, the effectiveness is really based on what army you're fighting. DR (in my opinion) are way better against horde armies (skaven, goblins, empire infantry army, etc.) then bolt throwers. If they scatter, they stand a very good chance of scattering onto someone else, whereas the BT still has to roll to hit and will only score a couple kills per hit on low point cost models.
Against monster armies (mino heavy beasts, demons, ogres) or knight armies (brettonia, empire cavalry army) , the BT is a much better choice. The table is less crowded, so scattering with the DR is not likely to hit anything, the BT still has to roll to hit, but if (when) they do, they are pretty much guaranteed to kill something that is worth alot of points.
Still...I'd prefer the DR. I just like the idea of raining fiery, explosive, shrapnel-y death on my opponent...even if it does miss them
