They're not the same.
In a server merge, sure you solve the problem of low population but you create new problems.
namely, some players will lose their names, some guilds will lose their names, and if too many are merged, you get queue times to log in.
Connected realms, is a way to achieve all the benefits of a merger, without the downsides. For one, if there's a player named Derah on bloodscalp, and another player named Derah on Maiev, and then these two realms get connected, neither has to rename their toon. The same rule applies to guilds.
Secondly, if they connect 6 realms via connected realms, you get to see many players with whom you can quest, raid, trade, and join guilds with, but still have the queue time of a low pop realm.
So in many ways, connected is FAR FAR better than merged.