|Found this over at BOLS|
Tie, same T, W and Sv values
The mawloc will come up as a large blast, S6 AP2 when it arrives, which averages to about twice a game. Being (very) generous and assuming hitting 5 models with the blast, considering to wound and cover saves, it comes to 2.75 per blast, so 5.5 wounds to MEQ a game.
A trygon charging with 7 attacks, rerolls to hit and S6 will cause 5.1 wounds to MEQ, and that is only in one round. Now it may take a turn or two for the trygon to even get into combat, but any round after the first surpasses the mawloc.
Granted the Mawloc has 3 base attacks on a TMC, so it could raise its kill count up a bit, but the trygon has a basic shooting attack, so we'll call that a wash.
The mawloc will almost always be able to blast in on turn 2, and with Hit and Run has a pretty good chance of getting to hit again later in the game if he lives. What he can't do consistency is hit models. Full 2d6 scatter can easily see him in the middle of no where the turn he comes in.
The trygon can struggle to get into combat, which is the whole reason for taking him. The opponent can just play keep away (though fleet helps to negate this a bit) and has a good chance of just shooting him dead if they are overly concerned. While he can deep strike in, it isn't a guaranteed that he will come in, so isn't considered as reliable, though most of the time safer.
The Mawloc has the amazing ability to show up anywhere on the board, and do damage. On top of that, he can move opponents models, which isn't a common ability. He can also re-submerge and come back elsewhere on later turns. This makes him very strong at objective/table quarter grabbing. His ability also allows him to circumvent Greyknights warp quake power, and getting inside their zone faster.
The Trygon also can deepstrike, and avoids mishap much like a drop pod. This can allow him to appear behind enemy lines and force them to deal with him lest he be able to assault at his leisure. But he can also march up the board, running and assaulting with speed thanks to fleet. This can force target priority on him and thus save other units.
Overall, the Trygon has a lot more damage potential than the mawloc, and if you already have a way of hitting your opponents backfield by turn two (Flying hive tyrants, ymgarls, spore pods etc) then the mawloc may not be needed in your list. One huge benefit of a mawloc though would be to counter the popular idea of DA with a banner/forcefield in a landraider surrounded by support. One direct hit from a mawloc could easily remove the land raider and all contents from the game. But this is situational, and I prefer to build towards consistency.
So what are everyone's opinions? Is the unknown damage from a mawloc more fearsome than the rapid approach of a trygon or two down field?