Answering the OP:
FWIW, I prefer a clip like the ones on these reins:
http://cgi.ebay.com/BLACK-RUBBER-REINS-WITH-CLIP-ENDS-4-BRIDLE-Free-UK-P-P_W0QQitemZ300207537891QQihZ020QQcategoryZ47274QQrdZ1QQssPageNameZWD2VQQcmdZViewItem?_trksid=p1638.m122
because the other clip mentioned here, while fine for longe reins etc, is a bit bulky AND the finger part of the snap can poke some horses unintentionally.
I prefer to buy the reins with the clip already sewn in, although I have pairs with clips I’ve added.
For whatever reason, adding a clip to a buckle end/hook end rein makes a different feel on the reins; although it is only a small weight difference, if one is trying to ride in such a way that a few ounces of ressure can make a difference–then the few ounces represented by the extra metal can and will make a difference. More to some horses than others.
IMHO.