First picture looks out of place, mostly because of one reason. You might know that all lines in a picture that point back into the center of the picture, actually point towards the horizon. It's what makes the picture look like 3D. You selected the background, and zoom-blurred it into the buildings instead of into the horizon. That's why the car seems to be in a completely different world as the background is, because the depth this way doesn't make any sence. You should always point radial blur towards the horizon in order to make the car fit in the picture. Because the lines of the car are also in connection with the horizon, the whole pic should remain like that.
With blurring in photoshop, you can just copy the background/original layer, and then blur the picture. Then you can erase the blur on the copied picture wherever you want, and maybe completely erase the picture if the blur doesn't look appropriate at all. It's an undo function thats always there. I always copy a layer before I do any adjustments, so that when I have messed around for too long, and it looks like crap, I can still delete that whole layer with everything I did to it, and get back to the pic I had when I copied the layer.