Well, let's do some math. Each brother you kill gives you a 1/200 chance for a piece of their armour.
So if you only killed guthans, you'd have a 0.5% chance to get a piece each time. And the average piece (total / 4) is worth 3.75m
Now let's set a goal of 10m profit. In order to get a total of 10m from only looting guthans (not counting runes/bolt racks etc...) you would need to open 532 chests. (10m / 3.75 = 2.66 * 200 = 532)
Now if we use the same 10m goal, but this time killed all 6 brothers, how many chests would we have to open?
Well the combined cost of all the barrows pieces is 22.2m. Divided by 24 potential pieces is 900k per piece on average.
With a 1/33 chance of getting a piece if you kill all 6 brothers, and an average value of 900k, you'd need to open approximately 366 chests. (10m / 900k = 11.111 * 33 = 366)
The next question is how much faster do you need to be able to do guthans, to make it more profitable?
Well if we assume that it takes about 20 seconds per brother kill, and let's say a minute to run the tunnel on average. Then doing guthans only would take you 80 seconds per chest. So 80 seconds per chest, times 532 chests is 42,560 seconds. Divided by 60 twice (minutes then hours) is 11.8 hours to make that 10m.
If we look at the otehr option, it would be 6 brothers, times 20 seconds per kill which makes 120 seconds. + 60 for the tunnel is 180. Plus 10 seconds of running between all the hills... So that's 190 seconds. 190 seconds * 366 chests is 69,540. Divided by 60 twice (minutes then hours) is 19.3 hours.
If you are able to kill the brothers faster, then it makes it a bit closer... For example, if you could kill each brother in 10 seconds, instead of 20, then it would be 10 * 6 + 60 + 10 * 366 / 3600 = 13.2 hours.
TL;DR Unless you can kill each brothers in about 8 seconds or less, farming guthans will be more profitable in the long term.