I use the Marshall by preference. It's 40mm Discarding Sabot rounds are so fast they have the ability to kill a MBT at over 2000m on frontal armour (multiple rounds).
As for it's speed, The Marshall will cruise at 109 kph on bitumen roads and 80 kph across country.
The Marshall is an Amphibious Vehicle, Hence it cannot be weighed down by Armour that will sink it.
As an ex-Cavalryman, Perhaps I grasp the Tactics of Armour, eg- I do NOT just drive straight into the center of an AO and hope there is no AT around.