Someone mentioned hitch weight capacity of the Highlander... it's 500 lb, not 350.
I've towed with my non-hybrid '08 Highlander for over 140,000 miles. It tows fine and has held up well. As for the hybrid, like others I would be concerned about heat problems with the electrical system. Also the electric rear wheel power of the hybrid's AWD may not perform as well on the boat ramp (the non-hybrid would be better suited, as its rear wheel power does not come from electric motors and its usual front-rear power split is close to 50-50.
Getting rid of a good vehicle and buying newer, just to gain a few MPGs, tends to not work out financially as well as simply keeping the existing good vehicle. Of course, it would be totally different if the existing van were becoming unreliable.