What is the primary difference? Just the engine used? What about transmission? Are they the same in both models? Are there any chassis difference?

 

Engine and brakes as far as I know

 

There are 4 different transmissions used in the 2nd generation IS (IS250 MT, IS250 AT, IS250 AWD, and IS350 AT). There's also a difference in brakes, suspension components, fuel delivery, VDIM and tons of others things between the 250 and 350. There are a lot of differences - look through a parts listing and you'll see literally hundreds of differences.

Javier

 

As far as the actual chassis and body size, they are exactly the same right? I've got people saying that the IS350 is bigger and wider than the IS250.

 

No, the dimensions are exactly the same between the 250 and 350.

Javier