Both places are good. The biggest factor is how much daylight are those locations receiving when fishing them. I specifically targeted walleye in the lower portion of Lake Powell last May. Here is what found. The best action was early in the morning before sunrise in areas adjacent to deep water, such as either side of Padre Point. When the sunlight hits the water, the walleye bite stops. If the sky is overcast, the walleye bite is extended a couple hours. Most walleye were caught between 3 to 30 feet deep. In the middle of the day, I picked up walleye in the narrow slot canyons where the sun never shines. The most effective lure was a 3 to 4 inch long chartruese grub on a 1/8 to 1/4 ounce jig head. Putting a piece of nightcrawler on a lure converts the lure into a magnet for walleye. Walleye are like catfish, they like meat. You won't catch many catfish on artificial lures. You won't catch as many walleye without using meat either.
The visibility of the water in Lake Powell is very clear. So it is helpful to use the most invisible fishing line known to mankind so the fish cannot see it. I use 8 pound test clear flourocarbon fishing line.
To target walleye, I launch my boat at dark thirty in the morning and head to Padre Point. I fish the east side of the point first because that is the side to receive the sunlight first. When the sun comes up, I then fish the west facing side of the point which is shaded. If the sky is overcast, the bite may last until 10am, if not, the bite may shut down around 8am.
This spring I am planning a walleye trip to the upper lake the first 2 weeks in May. My plans are to take my travel trailer and bass boat to Halls Crossing Marina. Then make day trips out of Halls Crossing Marina and spend the night there at the RV park. I have never fished the upper end of the lake yet. I will be monitoring this thread for tips on where to catch walleye in the upper lake the first half of May. Thanks in advance for information and I hope the info I provided on the lower lake is helpful.