This linked thread has some useful information about the Classic Era (max level 60, Phase 6/6) realms. Both Classic Era and the wotlk progression realms have identical server names. Some key takeaways are:
- Realms are clustered according to type (PvE or PvP) and timezone (Oceanic, West Coast, East Coast)
- Some realms receive free transfers
- Instanced PvP (Battlegrounds) combines all Classic Era clusters together so queues will be identical no matter where you choose to call home
My advice is to roll a lv1 on any prospective cluster and chat with the locals to get a feel for each communities’ culture.