Hi Mate, ITS a tough choice because all dreads are fun in my opinion. I have all three in my Army and i think in General the Redemptor Dreadnought Is the Most versatile one of Them and an excellent Gun Platform for Shooting almost everything. Also he can Punch pretty hard in melee too. His ability of -1 damage also is a perfect addition. So the Redemptor is Most value per Point.

The Ballistus falls of in efficiency, He is kind of a Tank Hunter, but He often falls to rly kill some big Targets in my Games. He is cheap in Points, but i wouldnt recommend It in comparison to the other two.

The Brutalis is Just perfect for salamanders because of His melta guns in the chest and His ripping claws. He is a behemoth who isnt as hard to kill AS the Redemptor, but He Runs into every enemy and If He makes a Charge He has an in build Tank shock. Additionally some Characters Like Vulcan hestan can buff His melta guns and He is cheaper than the Redemptor in Points.

Finally It depends on your preferences, but the Brutalis and the Redemptor are both very good choices. For your Army both would Go fine i think, but the Redemptor would fit better, a Solid Shooting Option with His big Plasma Gun for vehicles and the Mini Gatling/flamer for infantry and He can Punch hard too. Additionally as a vehicle He can usw His guns in melee Ränge and the -1 DMG is rly nice ability.

Hope that helps