Depends on whether you want rock solid security or expandability. I am setting up a gaming station & home media server and chose the ASRock. For me, the difference was the 6 SATA 6.0 & 4 SATA 3.0 ports for expansion. The ASUS was lacking in that it has only 4 SATA 6.0 & 4 SATA 3.0. The ASRock has 3 PCI X 16 slots and ASUS has only 2 PCI X 16; not that I will ever use a third GPU. One more plus to the ASRock is the dual LAN compared to a single LAN for ASUS.
I considered the Sabertooth because of the heat monitoring and dissapation, but the lack of features of ASUS and cheaper price of ASRock was the kicker.
I have had ASUS, AOpen, MSI & Biostar mobos in the past. ASRock seems to be as good or better than the competition.
If you want a board for stability, durablity and performance either of these boards will do. The Sabertooth probably will last longer, but the question is how long before your next upgrade? My last personal build before my current one lasted 8 years (with a few tweeks) and is still running -- I gave it to my kids & grandkids. It was one of the first dual channel 3 GHz and is stilll very capable even today. It can't run the most modern games but it does run Directx 9.0.
As I said earlier, I like the specs of both of these boards, but bought the AS Rock Extreme6 because of its many extra ports. I run a very quiet, cool rig so the thermal armor seems to be overkill. ASUS sets mobo standards, but ASRock is probably the price/performance champ.
Perhaps I misunderstood. Did you mean that you are upgrading now for a board that will last 4-6 years? If so, then either of the two boards should last that long (but they will not be the most modern product in 4-6 years).