I prefer the ride of IFS over the SFA. I usually gross 12-15,000 lbs. and occasionally just over 30,000 lbs., so I qualify, I think.
However, if I'm a bogger or one of those fellers, I hear the SFA is the only way to go. That's not really my thing, though. I have exactly no knowledge of that type of thing, other than it looks cool as heck!
I've only been stuck bad once in the mud when I was hauling a load of logs and had to have a backhoe push on the back of the trailer to get me moving again. In that instance, it didn't matter if it was IFS or SFA - I was not going anywhere.