reps have more pvpers for sure but we are by no means outright superior, infact the imps seem to rule the evening ( they win more games thats all) while it's basicly pub v pub after midnight.
think imps have basicly 2 guilds that pvp one being full pvp (snx) the other being half pvp half pve (ns) with pugs ofc.
pubs have abit more guild wise, there is 3 full blown pvp guild ( mvp, gimp and TA) and some pvp/pve guilds who like to play now and then (excession, mindwalkers, corvus to name a few)
the bigger problem is if you think along the lines of " OMG MUST WIN EVERY GAME OR I'LL RAGEQUIT" or if you think all pugs are scrubs your better of playing on the imp side as the quality to quantity ratio from what i see is better then the pubs side.
hope this helps and i hope you roll a pub we always want more pvpers