Sand isn't really the best substrate for these guys (or for any reptile IMO) as long as it is the fine kinda that isnt real thick and granular; however, in my experience I've found it to just be messy and make the cage look gross. I mean, lets face it, i'm sure they would come across loose sediment in the wild and somehow they didn't eat dirt and the species survived.
One reason why sand is bad is because it pulls any and all moisture out of their skin that is always buried in the sand (their feet) thus making it very hard to shed skin on their toes. This also happens to be one of the most crucial areas for them to shed and I've seen LOTS of leos missing toes due to bad sheds on their feet.
Carpet is very nice, clean-looking, and will help you turn down your lizard paranoia one more notch. Just my .02.....