Well, I list few major reasons where WoW is better than GW and vice versa. It is possible to like from both, you know.
WoW
+ open and more alive PvE
+ good factions system (random human patrols are fun and annoying)
+ not so much AI to reduce human interaction
- heavy grinders are always step(s) ahead in combat
- combat is quite messy with large groups because there is so much functions
- monthly fee
GW
+ very large player community (WoW has like couple thousand per server?)
+ pretty balanced and simple combat system (skill matters more than spent time)
+ no monthly fee
- current AI mates are converting GW to single player
- PvE world is too private and quite tight.
- not much PvE rewards and fast dynamic action in both PvE/PvP
I'm not going to compare graphics because both have their strengths and weaknesses. WoW might look old but its cartoon style forgive so many technical flaws while GW is more serious and detailed. Artistic issue which is completely subjective.
IMO, GW PvP is designed for too organized players while WoW PvP prefer grinders and cookie cutter builds. Personally I like random and fun PvP action which has other goals than wiping the hostile team. Fort Aspenwood is the only working solution for GW because it is fast. Sadly, leeches are soon going to kill it.
WoW shines with random encounters and open PvE with better human player spirit. That is the major pro for it. Can't really think anything other where it is better than GW. However, this alone improves PvE a lot. Some like from expensive mounts and rare powerful gears but these shouldn't be advantage because it just attract 1337 n00bs. Sure, it adds PvE goals but these should be easier to get so 24/7 players won't gain too much compared to others.