For someone that does not care whatsoever about strength and only cares about adding size, which one is better?
10/8/6 - add a set of 15 at the end with 50% of your 6 rep max.
How about you lift heavy, get stronger, and get bigger all at the same time. It doesn't really matter which one is better. It all depends on what works better for you. You need to experiment and find out.
If someone cares about adding size, they HAVE to care about adding strength.
Make yourself a giant pile of hot steamers and lift it 35 times each night before bed, and you'll be huge in no time.
I care about strength because that is how I add size, but the end goal is the size.
I've always heard that hypertrophy is best accomplished with a rep range around 12-15 for lifters without a ton of lifting under their belt, whereas lifters with more experience and time under the bar get better size results with a rep range around 6-8. It also depends on what muscle group you're working. I'm not sure exactly how it breaks down for every muscle, but I know that hams and glutes respond better to a high rep range (12-15) whereas other groups respond better to a lower rep range and a heavier weight. The rear chain muscles, especially the hams and glutes, can handle a higher volume per week than other parts can. The general point is that different body parts respond to different rep/set patterns. There is no all-encompassing scheme that works equally as well for the whole body.
No bodybuilder uses only one rep range.
Example of a typical workout:
Flat bench 5x10,8,6,4,2
Incline bench 4x10,8,6,4
HS Bench 3x12,10,8
Cable flies 3x15
The rep range for hypertrophy is 1-20.
You find the best rep range for each specific exercise. You wouldn't do a final set of 4 reps for lateral raises would you? And you can go heavier than 15 reps on T-bar rows.
And that is besides the fact that most of us work out in a range anyway... for instance I might squat a weight I can only handle for 4 reps, but I'm going to keep adding reps to that working weight each time I come back to it until I can get 10 reps with it. Then I'm going to add weight again until it's heavy enough that I can only get 4 or 5. What "rep range" would I be in there?
The point I'm trying to make is that there is no answer to this question, except for you to find out personally for each of your staple exercises.