both zoner cruisers only have 2 heavy slots, so you will have to prioritize. I'd go for cerbs.
Missiles are only really worth it in tandem, and again that only works out if your ID permits you to use a faction launcher, so that you can have 1 basic missile launcher and 1 Liberty missile launcer (as an example) - that way you can carry double ammo (yes, missiles are ammo based, and you'll soon find that that sucks terribly). They can also be flak'ed, shot down with CD's and decoyed with CM. They have a lot of weaknesses that are that much more apparent when you only use 1 launcher.
Mortars are also pretty terrible in most cases, unless you are fighting in a group where you can sneak in a shot unnoticed, fighting something really big (like turtle-sized) or fighting someone who has no clue what he is doing. If you have a third heavy slot, however, it can be a useful support weapon and you can use it as a finisher. On a zoner cruiser, I wouldn't.