Shadowbolt + improved shadowbolt debuff (ISB) will generally out damage incinerate + immolate ..... this is pretty much the main diff between the shadow and fire specs.
There have been numerous discussions on this on these boards, and both options are viable. So basically play what you prefer
If you want to go into depth then search out the previous discussions, theres a lot of info in them, a lot is dependant on guild make up, group set ups in raids etc tho.