Fortify Sneak decreases your chance of being detected in general, while Muffle decreases the noise you make while moving.
Lmfao, great answer. Sike. Terrible answer. He meant specifically they obviously both help in reducing the odds of being detected but which is greater and what percentages do each give.
Actually, it is exactly that. The noise you make depends on your armor, so the heavier the armor, the more noticeable you are. This means that if you are in full daedric, which weights 81 (with no shield or perks), using Muffle reduces (I think) by 50% the noise you make, meaning the game will consider it as a 40.5 instead. On the other hand, Sneak can only be increased by enchanting by what... 30-40% ? So in this case, the effect of Muffle will be greater.
On the other hand, if you have little or no armor, the effect of muffle is far lower. Say you use full elven armor, again with no shield. That is a weigh of 7. In this case, not very usefull to reduce it by 50%, much better off with sneak.
Also, keep in mind that the sneak skills branch has Muffle in it, called Muffled Movement, requiring only level 30 sneak, and being required for some of the cooler stuff in the sneak branch. Better to use this skill, along with sneak as the first enchant on the item.
Pre-existing muffle enchants in the game have a Magnitude of 0.5(reduce sound by half) but custom made enchants have a value of 1.0(totally silent). It's pretty cool one can do that with a petty soul gem as well. However, just because you can't be heard doesn't mean you can't be spotted. If you wear a set of very strongly enchanted Sneak gear, you can move around people in fairly well lit areas(like outside during the day)without being detected. I don't use the restoration potion glitch, but I've read of people who enchant a single item with such high levels of fortify sneak that they can move around pretty much any other entity in the game as though they were invisible(with the added bonus of not being heard, either). Since Silence in particular is a perk in the Sneak skill tree on the way to a better perk(Shadow Warrior), I make sure to have a set of gear enchanted with Sneak for occasions when I just don't want to deal with stuff I'd have to deal with otherwise when I'm not wearing the set.