Generally with Achromatic refractors, the shorter the FL for a given aperture and lens contruction/material, the worse the chromatic aberation is. ie, 80mm x 800mm much better than 80mm x 500mm. The trade off is you have a narrower field of view with the longer FL for any give eyepiece, but higher magnification. Just depends on what is important to you. High mag, narrow FOV/Lower mag, widefield.
