I've done 2 different gaming rig builds using ASUS Crosshair mobo's. One was the Formula IV, and the other a Formula V. As said earlier, the construction and build quality is top notch. I have never had one seconds trouble with either crosshair mobo and have been extremely impressed with the performance. They are a bit pricey, but my Formula IV has been clicking right along for 3 years now without a hiccup in my own gaming rig. The Formula V is in BB's rig, and performs very nicely for 6+ months now. I've not owned a Sabertooth, so I can't speak to it, but it's an ASUS mid/upper end board so you probably can't go wrong. I started to put that into BB's rig when I built it, but I had such good performance from the Formula IV that I decided to shell out the extra bucks on a mobo I felt I knew would perform well and last.
When you compare the two, you get better audio with the Crosshair, and better over clock ability and a wider range of the higher end (faster) ram that you can use. If you are not into overclocking and such, the sabertooth (at least on paper) is a great board for the price. I'm just stuck on the crosshair because I know it performs very well. If you need to save 50 bucks, the sabertooth sounds pretty good.
...and on a side note.. I originally had 2 Radeon HD 5870 2GB vid cards in CrossFire in my own gaming rig. To save money when I built BB's, I pulled one out of mine and put in hers. I had originally planned to eventually upgrade my vid cards and stick the second 5870 in her rig. However, I saw NO decreased gaming performance in day to day play by going with one card. I'm sure that if I ran a high end performance app my score would be lower.. but who cares about what your performance score is if your real-world gaming performance isn't affected. Crossfire and SLi are nice, but I've not seen where it's necessary with the higher end vid cards that they are putting out today. Maybe I would if I played more graphically intense games.