They are such completely different kettles of fish I would massively suggest you either;
>Proxy, pretend you have one and play it and see how you feel about it a lot before putting it together
> MAGNETS- make it hot swappable between the two.
tactically its apples and oranges.
The maulerfiend is kind of like a dreadnought missile, he zooms straight at something..He will kill a big points heavy tank like a Landraider- he might struggle against something with lots of wounds.. Then you send in a support squad to mop up (/ hope the thundernaters piling of landraider are dizzy and forget to butcher the thing).. Maulerfiend with melta= 1 shot queen sacrfice kind of thingy.
Swapping the melta for the lasher tendrils reduces peoples attacks by many many, as they stack.. so anyone attacking you in close combat (in b2b) has -2 to their attacks (to a minimum of 1).. making him much more survivable if he just pelts into a mass of troops.The problem is if he's not in close combat he's very close therefore he will get cut to piece by melta (et al) weaponry very quickly.
As a demon zooming with his special zooming scuttle he's quite handy for your troops to follow behind for mobile cover (which he doesn't need due to 5++ and IWND)
The forgefiend is a more passive sit back and shoot guy who stays out of combat sitting near the back of your army (36"s) chucking out 8xstr8 shots. Thats good for getting rid of light armour and monstrous creatures with not great saves. He costs about a third more on top.
Plasma variant was originally seen by peeps as awesome, I can't remember why..It has shorter range committing you to the middle of the board and is only 3 shots if you pay more for the (ok so blast and ap2..but its nearly landraider points).
From the gut I'll suggest autocannon version of forgefiend, just because tactically its easier..but I'd strongly suggest against committing yourself before you have a bash with each
(I'm just a cheap wad who fills his heavy slots with 5 havoc 4 autocannon
)