Well I disagree from the point that Mastiff/Astarte have only 3 system slots compared to the third MS. Have you tried the Alex campaign? There are so many Berserks in the last mission that Mastiff would last as long as a snowball in hell. But the same aplies to Astarte
But it is just a game. If you are fine with the Mastiff, then no problem
Everyone has a different playstile. I prefer to lure enemies to fire on my MS and then let fighters to take them. And for this the Astarte seems better.