I would get an analog (not digital scale) because if you run out of batteries, it's a pile of junk. Analog scales, if taken care of will last a loooooooooong time. Different scales are built with different limits, just as a convenience to consumers, don't get a 100lb one if you are never going to measure that much.
If the 100lb scale is the same size as the 25lb scale, then I would beg to bet that the 25lb scale would be more precise in it's measurements (or the face of it would have smaller denominations so that you can more accurately read the scale face).
Hope that is useful. If it's digital....I don't see a reason not to get the 100lb one if it's cost is comparable.