Looking at the specs, they all have the same 276ci engine. The 6403 is a larger framed tractor compared to the 5000 series. The 6403 is the base model of the 6000 series. The 5525 is a better equipped tractor than the 5603, which is a base model of the 5000 series. Just depends on what you want to do with the tractor. The 5000 series are more compact and should weigh less than the 6000 series.
The 6400 I have has been a real good tractor. There were not the variety of models available when I bought it in '96. The 6430 would be the equivalent to it now.