I'd say the Ute is great if it fits you, the single size is the major weak point to me. It is nice for lighter duty being a bit shorter than the xtra.
The major pro for the Xtracycle/Big Dummy, I'd say is the unrivaled versatility of the freeloaders. With a couple reusable grocery bags or lightweight stuff-sacks kept in their inner pockets they will carry anything you can stow in six panniers, and so much more. Without some DIY solution, the Ute would require much laborious lashing of many things swallowed quickly by the freeloaders.
As far as your concerns about light off road use, the Big Dummy's offroad prowess is well proven (
www.ridingthespine.com), but the Ute should be fine. I'm unaware what the clearance is like on the Ute, but they do make 700c big apples in 2.0 and 2.3. There's also plethora of knobby 29er tires now, and a whole spectrum in between knobby and slick. If you feel too high up on the Ute, that could make handling a little more difficult though.
I think Xtracycle even now has some 16" big dummy framesets, though you're looking more at completes.
I'm guessing that the benefits in torsional stiffness between the big dummy and a regular xtra conversion become less noticable to smaller riders. have you gotten a chance to ride a regular xtra?