Interesting that before asking for Race was considered "pointless" in MMOCstickies even though there were clear differences. In WoD the Differences are minor compared to now but the MMOC Mods let it be.
OT: PvE Gnome for Arcane and Worgen for everything else due to ther movespeed Racial.
However, the differences in DPS for WoD are minor as previously established and you should seriously consider the gameplay fun. Like for Example can you live with Marge Simpson Sisters' voice if you pick female Worgen, or, if you follow it through, with taking the lame Kindling Talent as fire instead of the cool Focus Crystal or Meteor which probably have nice visual effects.
€dit: Pandaren have a flat Stat bonus so they might pull ahead in the first Raid tier. They are also smooth to level with their rested Racial. Don't know how relevant this is, but Kripparian did #1 World DPS on Elegon on his Pandaren Hunter.