Well, a shart means you started farting. People dont usually let one rip in front of a group of people, not counting vulgar idiots who think its funny (looking at YOU kaz65). Chances are you'll at least have a chance to inconspicuously get away and clean up.
Snarting starts as a smeeze, which one generally does in front of people, with no real thought, so, a snart means you'll end up ripping one in front of everyone.
Sharting is worse for what it actually is, but snarting is worse in that you're more apt to be seen doing it...