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If I was you I’d buy a subscription then after payment is made cancel it so you can play for a month, otherwise you would need to buy 2 months game time. Then check the game out.
Classic realms are connected into 3 groups, PvE, PvP1 called the Firemaw Group and PvP2 called the Gehennas group. Playing on a PvE realm doesn’t matter due to the connections, but one PvP group has more players than the other, the Firemaw group, I believe.
The connections mean that it’s like one big realm for all those in it, a shared AH, guilds everything. If you send anything to anyone in the game the name format is Danthetank-Morgaine, that’s about the only thing that’s different from a normal realm
I say big but really the population isn’t that large, a few thousand at most, and most people only raid log. No doubt someone will be along to disagree with me sometime.
If I was you I would choose TBC realms as within a few months, we don’t know exactly as it could be a few more, Classic Wrath is coming out. Most peoples guess is the pre-patch in a few weeks to September as they are starting the XP boost, which thy call the “Joyous Journey” a few weeks before the pre-patch.
Since summer ends in September, the 1st or the 21st, depending on which one you use, it could be as late as that.