Snap Shackle vs Carbiner

e.g.
http://tinyurl.com/53ntlv
     VS
http://tinyurl.com/4h8wa9

Which one would you rely on in a situation where:
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1) The shackle/rope attachment may be flopping 
   around.
 
2) The connection is not constantly taut or under 
   tension.

3) Having the device come loose spontaneously is 
   a beeeeeg no-no.
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PeteCresswell