Flaming Gorge is closer to the northwest, say Washington or Oregon. I live in Salt Lake and it takes me 3 hours to get to the "Gorge". Lake Powell takes about 5 hours to Bullfrog and a little longer to get to Wahweep. Lake Powell is a warmer lake also different fish, lots of large and smallmouth bass and tons of stripers or striped bass. Lake Powell has no trees for the most part, but very beautiful. Lake Powell is approx 200 miles long. Temps get over 100 in the summer. Lake does not freeze for the most part. Outside temps in winter get to freezing.
Flaming Gorge is approx 100 miles long. Trees on the Utah side of the lake, sagebrush on the Wyoming side. Big fish Lake Trout 50+ pounds, smallmouth bass, trout, salmon (kokes). Temps in the summer 85+, winter time the lake is frozen. Temps in the winter -25 some years.
You can kind of compare the two lakes, both are redrock as far as the eye can see. The structure of the Gorge is a little more jagged. Lake Powell is more smooth. Both are great lakes so I would just have to say go to both you will not go wrong. We usually go to Powell until about May then spend the rest of the year at the Gorge until ice up.
I hope this helps. John