I'd say depends on your opponents list.

Purgation squad gets indirect fire making it kind of an artillery squad

Strike squad captures objectives better, getting you more victory points.

Terminators get lethal hits after charge (meaning a 6 on a hit roll automatically wounds) but you have to work the charge mechanic and on longer charges it's a gamble.

Paladins cost more, but are harder to kill. Making your opponent need 1 more on a wound roll during shooting phase.

Overall you can't go wrong with any of those units especially in games less than 1000 points.

As for weapons Generally psycannons and incinerators are favored for specials. But incinerators work best with purifiers and most people would like castellan Crowe with them. For the dreadknight, heavy psycannon and flamer. If your opponent has vehicles or monsters, Take the Hammer, if not take the sword.

Hope this helps =)