the left one is much better, the white outline adds nothing. I'd darken the little antenna bits coming off the roof, they look a little foregroundy right now. The bit of armor extending over the treads looks too flat and weird - the shading is rounded, questionably, and there is no indication of thickness or volume to it. The blob on the side of the tank seems a little soft and confusing in shape and purpose. I'd like to see the front of the armor, between the treads there, have some more details, it's a little dull right now. The treads are pretty sloppy, both in color application and general shape and size. The interior of the wheels need to be shaded darker.
I like your colors, and I love metal slug, and generally speaking your lighting and shading is solid; only the antennas, wheels and treads strike me as problematic renderings in that sense. The dithering is well done, and it adds a weathered texture to the surface that works very well. I think its mainly the linework that is really lacking in this piece; if you fix the lines for the treads, the front armor, the blob on the side, and maybe a little work on the main gun, you will have a pretty solid piece here!
