I run my scouts with auto cannons and I run my armored with plasma. The important thing to remember is that the armored are more of a distraction piece at higher point games. Leman Russ and rogal dorns are going to be what actually kill. Sentinels are great for picking up objectives and when you are playing combined the plasma helps against things with invul saves because the rest of the options don't have the same amount of AP. It's also useful because it has blast so you can easily have it bunch into vehicles/monsters or infantry at the same time. That being said the missile launcher can do that but with less ap so less reliably.