We have driven thru SLC with both the RV and boat multiple times. Although traffic can be bad, it is no where as bad as a number of major cities. (We just towed my son's 25 thru LA which was stop and go from San Fernando to San Clemente--over 80 miles...)
We like to launch at Wahweep, but it is a bit further than to Bull Frog. A lot depends on where you want to go after the visit to Powell. If you want to stay North, then Bull Frog. If you want to go I 40 or I 10, then Wahweep. Same if you want to go to Vegas, or LA etc.
We leave I 15 at Scipio, and take 50 toward Salina, but take the cut off 24 toward Aurora. Hit I 74 toward Richfield, and take 89 S at Sevier. Keep on 89 (not 89A) at Kanab to Big Water and Powell/Page. This is a scenic route, going near Bryce and Zion. Along the entire route there are many interesting stops if you want to take the time.
Have a great trip, and looking forward to your report and photos on cruising Powell. Either route is great. If you go Bull Frog, do any provisioning at the Walmart at Price. On the lake or near the lake fuel prices are higher. Not positive about diesel prices on the lake however.
We like the lower lake better than the lake above Bull Frog. I also think it is worth the time to go to the San Juan Arm. A lot depends on the amount of time you will have on the lake.