Generally speaking, 8-12 is ideal for muscle growth since it's the middle ground between general strength and endurance. Muscle fiber size is based on thickness of filaments, which actively contract, and the endurance stuff like fluid, carbs, enzymes, mitochondria. 5x5 is actually a strength program, not a hypertrophy program.

Things get a bit more complicated though since people are finding that certain muscles tend to respond more to certain ranges. Mostly based on whether they're mostly endurance fibers or strength fibers.

The outer two muscles of the quad have slightly more endurance fibers than the middle quads. By adding high rep training you can make your thighs wider.

I don't know what's best tbh. If I wanted to build huge thighs, I'd maintain double leg squat as my main and just add reps. But I'd also do reps of single leg work. If you feel stable enough on one leg you can do something like this:

4x5 squat (adding reps as you go) 2x6 single leg exercise based on 666 progression

If you aren't stable enough you might have to do high rep training with your single leg.

I'd basically progress into a simplified sort of undulating periodization:

Switching between each phase every month or two. If you feel like things are getting stale you can add variety after a while.

Here is a little more food for thought:

Elliot Hulse talks about using dropsets to make huge thighs... Dropsets just build huge muscles because they work all things.

The VMO is slightly slow-twitch dominant, the vastus lateralis is approximately 57% fast twitch, and the rectus femoris, the center of the thighs literally and metaphorically, is a 65% fast-twitch powerhouse.

Muscle Specific Hypertrophy: Biceps, Back, and Lower Body