Extra Line vs 6 atk

This comparison is all assuming that you can achieve 9/6/6% on the second belt. How reasonable is that? Or like Jellyflower said, how ambitious are you?
In some ways, 6 extra att in the hand (as it were) is worth more that the possibility of achieving 9/6/6 with great expense or luck.
Why don't you keep the high att one as is and start cubing the low att one. If the cubing goes well, use that one. If not, or you run out of nx, you can sell it with 9/6/garbage and still go back to the high att one.
Simple probability would suggest that 9/6/6 is very expensive to achieve, but you do see some around, so either ppl are spending on cubes like mad, or the programmers made things a bit easier than they might have been.

Yea, atm I'm leaning towards the 10 atk seeing as how I've used quite a few cubes myself and never once had all 3 lines stacking with a stat, 2 is rare enough . Also, if I cubed the 4 and finally got all 3 lines to stack with STR but it came out to 18% the 10 atk would win with 15% (which probably would cost fewer cubes) and I'd be sad lol. I still have to focus on getting my top/bottom/shoes to 15+ str as well anyway.

Edit: I tried "junk scrolling" from what my friend told me (his ritual is fail 3 60s then use your scroll) and the chaos failed so its 10 atk still haha :/