While I think that Mechwarrior 3 and Heavy Gear 2 are different enough from each other that they don't hit head on, GU makes them do exactly that an compares both of them.

The snow in the early missions of Heavy Gear 2 is quite impressive, as are the rocket trails and smoke from anything destroyed.  Bumpy terrain, with lots of small rises and dips, is handled quite easily in the Dark Side engine.  Something Mechwarrior 3 doesn't have, space combat at zero gravity, is also handled well, though it tends to get very taxing on the graphical side.

Unfortunately for Heavy Gear 2 fans, the price paid for this visual impressiveness is too high.  Mechwarrior 3 accomplishes rainfall, smoke from destruction, rocket flight, and even steam when the lasers hit water, for the small price of a 2 MB video card.   Further, while I was stuck in 640x480 resolution in Heavy Gear 2, my 8MB Riva 128ZX video card (yes, I know it's old) was able to render every bit of rain, fog, smoke, and miscellaneous effects like sparks in Mechwarrior 3 at 1024x768 resolution without complaint