Ha, nice! I mean, "onomastics" really should win simply for already being the established term. But if one were picking the name from scratch then I'd say "nominology" has probably...

    * greater evocativity (at least for us philistines)
    * nearly equal brevity (same number of chars, one more syllable)
    * equal greppability and googlability
    * greater spellability, maybe (again, at least for us philistines)
    * equal pronouncability
    * equal verbability (nominologize/onomasticize, nominological/onomastic, nominologist/onomastician)

Of course "nominology" mixes Greek and Latin roots so that's presumably fingernails on a blackboard for more educated folk than me. Cf http://messymatters.com/nominology/#comment-530036107