My thoughts: excellent model. The night render is amazing. Be a pal, and let me in on how you did that! Is it just grain and glow? Which effects are part of the render and which follow it?
The most room for improvement I can see in the day shot is with texturing. Needs more dirt. Treads on the tires. Maybe some more color variation on the top spinny-thing - some parts black, others silvery-metallic. The glass on the cockpit sticks out as a little odd. Unfortunately, the only fix is probably to model the inside and to make the glass partly transparent. What you've got is a good attempt at hiding it, and I imagine it'll look even better when put in action, the content of the reflection more varied. For now, you may want to turn down the glow/glare a bit.
I'm kinda confused by the specular highlights in that there are two pronounced ones, but seemingly no minor ones. I mean the white flecks, one on the cylindrical thing in the front and one on the wings in the back. Are these the only shiny surfaces on the vehicle? Why isn't there a highlight on the main wing? It seems like with the amount of glare coming off the window, most of the surfaces should have one. Maybe you need to bevel some of the edges? Maybe you only wanted to fake those two for the night shot?
Great work; hope this helps.
Thomas