I reckon, for solo play, you want the war-hawk.
Even with 5 mantis on you it is not so hard to just keep out of range and pick them off one by one, and it would be quicker than doing the same thing in a tank due to the availability of rockets.
However, for group play, I would say a tank would be equally matched for its usefulness. I've only got as far as the Gemini system, and if it gets increasing more difficult as I suspect it will, then a squad with a tank, an engineer and a 'fighter' or two would make a lot of encounters painless if the tank gets the aggro and the engineer keeps him alive.
I expected to see more play in that 'holy-trinity' style, but up to now I have mainly seen players going solo with a fighter.
In terms of 1-v-1, if the game is properly balanced then a war-hawk should kill a tank at the same time the tank kills the war-hawk!
