I never recommend taking two lane roads through the Res and while a bunch of that has now been converted to 4 lane or has some decent passing lanes it is still slow, with RV and boat traffic to Powell and local drivers who seldom get over 50 mph
. On the two lane sections there are very narrow shoulders so should you have a problem and have to pull off you will be in the bar-ditch. Top your tanks in Flagstaff so you are not forced to fuel before Page. Fuel can be expensive at some stations. There are a few around Cameron that may have better prices.
While 93 might be bumpy I actually don't recall that being an issue and I travel it fairly often to Vegas. I have to admit I am not pulling a trailer when I do so that could make a difference.
If you do decide to take 89 I suggest you plan time to take 89A and visit as many of the national parks in Utah on the way to I-15. Be aware that there are long "climbs" on either road. One topping out on the Kaibab and the other on the route into Page.
One other issue that came to mind with May and a trip. Check weather reports before taking off on either route but especially 89. There is nothing, and I mean nothing to stop the wind across the Res and it can be a major factor for high profile vehicles.