Just among these two for 4 adults and 2 baby seats? I'd go with the Highlander. I did some research about these two models before and it turned out that the Sienna is bigger and has more convenience feature, but in terms of safety, minivan has a natural weakness compared with (even just) mid-size crossover SUV. Highlander ground clearance is about just 1.5 inches higher, which gives quite some advantage involved with minor accidents. For bigger accident, I'd go with Highlander as well - although Sienna is much bigger, its weight is actually about 150 lb less than Highlander. For mid-size SUVs, Highlander seems to be the most recommended model; for minivans, Honder Odyssey seems to be the most recommended model. If you have to buy a Jap car, I think you should compare Highlander and Odyssey...