Are you going to Yellowstone too?
We have taken both routes with our trailers (and many other times in a car). On 191 between Pinedale and Jackson, we stayed at the very scenic Granite Creek USFS campground. If using 26/287, we stay at Colter Bay (our favorite campground in Grand Teton NP) with its incredible sunrise photo op of the Tetons reflected in Jackson Lake.
If I were only planning to be on those routes once, I would choose 26/287. Much more scenic between Lander and Jackson than the segment of 191 from Rock Springs to Pinedale.