Rendering Time compared with two pics?

Here is the problem
I have two models.
one has 3,5times less verticles and faces,has no OSA enabled,picture size is 2 times smaller and it takes 3-4 times longer to render that picture.
Why?

What are the materials on each of them?

Materials can make render times longer depending on what your doing. For instance reflection and transparency, both take longer. Also any material with NOR, Spec etc. can take longer to do the calculations.

ambient occlusion takes a long time too.

The octree resolution will seriously influence your render times.

the one with smaller number of verticles has got very simple materials and not much reflection

the other has multiple image textures+blender textures,nor,ref,spec,hard and raymir enabled

How about posting the pictures (or the wireframes)

Here are the pics
the plane pic took about 8min 20sec to render
http://img518.imageshack.us/img518/3228/jet820min2ss.png

the other pic(couldnt wait until it is finished)took about 1h30min
http://img527.imageshack.us/img527/6831/abrams15h1ki.png

So…what might be the problem here

There are perhaps 300 track parts so it could have to do with the
reflection and the track parts.

I dont think it is caused by tracks and reflection.
THAT bit difference.The plane picture has no empty space,but the tank pic has allmost 1/2 of the pic empty

ray mirror is extremely slow. only use it if you must! Turn Ray off in the render panel and see what your render time is like. probably less than 20 seconds…if you have a decent computer.
If all you want is a shiny effect, increase specularity and hardness.

Blenders renderer seems to calculate all faces when rendering. Regardless of whether or not they are visible from the camera. I noticed this when I was doing my battleship. If I moved meshes in the render that were not visible from the camera I got a tremendous speed up.

What kind of system are you using?