Aion is a has a great gameplay. Superb interface etc.
AS to WoW, i´ve played that over a year now
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Both of theese games are great, somewhat different. But yet the same. If youre gonna like more intensive pvp, go to Aion (or go and create a character on a pvp WoW server, youre choise), if you want more of questing etc. Exploring, go to any WoW server.
I could go on for days about this matter, it coems down to a simple matter what you like to do.
I would recommend both games for anyone that has got alot of time to spare.
As for ppl say that WoW players have no life, it comes down to how mutch you play it. Theres So So SO mutch to do/explore/get in WoW, even players that have played it for 5 years havent dont it all yet. So yes, you CAN loose youre life there.
The same will go for Aion tho, but not that there will be so mutch to explore/quests to do. But for the great look at the game etc (damn, here i start again =P).
Choose vicely, you haven´t time to play "hardcore" on both.