I don't care much about durability, I only care about how well a string performs. Also I haven't had any of the 2 long enough to test durability.

Lethal 66: Strung at 26x29 on a headheavy stiff flex racquet (Lethal 100) When strung with bg66um the power generated was simply insane. The Lethal 66 string does not generate anywhere near as much power. The feel is also very hard, on drops it felt like you were hitting it with a piece of cardboard. Feels like a 0.8mm string. Does everything average (at best). Basically it is absolutely not worth the extra 15-20$ to have your racquet strung by apacs with this stuff.

Stern 9: Very similar to Lethal 66 except the drops are even worse in terms of feel...felt like hitting drops with a tennis racquet. Power is very similar even though Stern 9 is slightly thicker than lethal 66. Once again, not worth the 15$ extra.

In conclusion: Basic bg65 is better than both these strings, I am really not impressed with these Apacs strings.

Remark: If you have your racquet strung by Apacs you get to chose 21x24/22x25/23x26 etc (cross always +3) and yet they string with 2 knot...so your tension transfers over in corners. I love Apacs and their racquets but I was VERY disappointed by their strings and stringing technique.