Look up barrage in the index. The explanation is in the illustration on the left side of your book. I fail to remember the page number. I think it is under the blast weapon section.
I don't run either since I like to be mobile. However if I was to pick I would use missiles due to having more options. However in a non tournament setting I would run mortars, problem with template weapons is they slow down the game.
For Heavy weapon squads 3x Autocannon are probably the best all round choice. Cheap, lots of high strength shots.
Missile launchers pay a really high points premium for their flexibility.
Mortars dont do anything that cant be achieved more points effectively with other units (this is the same problem with the lazcannon option as well).
I do like the mortars though, and find 3x squads around the 'sweet spot' for doing real anti-infantry damage with them. The problem is that for the same points I could have 30x guardsmen (who will have better survivability and better anti infantry, even without "first rank fire"), or a Valkarie with 2x missile pods.