That the 2.5-20 is more critical of eye placement ("tight eye box"), more finicky on parallax settings, and shallower depth of focus. Less than an ounce lighter, probably not real good on 2.5-4x anyways with the ffp reticle. So with the 4-32 even if you only ever used it as a 4-20 you have more forgiving mounting situation with longer tube, more forgiving eye box, parallax, and depth of focus.
That is what I've gathered here and elsewhere from folks who've used both so take my input with a grain of salt but I'd recommend reading folks' comparisons. I've had my own disappointing experiences with high zoom ratio scopes jammed into a short form factor and it makes sense that the 2.5-20 would have similar issues.