Biggest advantage of the Casita is the larger front bathroom. Some like the full fiberglass underbelly. The optional lift and larger tires
are better for back country use. The galley is tiny. The side bunk option gives up the large dinette window.
16 (layout 4) has more headroom (6'2" vs. 5'10"), a lot more galley space, and better ventilation (back window opens). It will probably be a bit lighter as well. But the side bathroom is small and has limited headroom.
offers roof AC; Casita installs a window-type AC through the front corner. The former is more efficient for cooling, but the latter reduces overall height and drag. Scamp lines the shell with marine headliner over foil bubble insulation; Casita uses residential-style foam-backed carpeting. Scamp supplies (very thin) curtains; Casita provides mini blinds. Lots of little differences.
Raz's advice to get a look at both is good. Things like headroom, bathroom size, beds, etc., need to be experienced in person. The main dinette bed is the same in both at 45"x76" and some find it too confining for two adults, while others just call it cozy.