if you are a little bit talented you can take any of the kxf al-frames.
the 450-frame has a little bit more weight and is also stiffer as the 250-al-frame. thats all...
i for myself prefered the steal frame conversion, because i had this frame sitting in my garage and at least it was easier to do. welding steal is also easier as welding aluminium, but with enough money and the right technology, you can build and weld anything to an great bike or whatever...