As someone who played the first few months of Rebirth and has been playing the last year or so on ED here's my little pros/cons list
Rebirth
I don't have experience with raid content there, the top-end population simply couldn't support it horde side when I was playing.
Pros:
Fixes came quick. The server was
very broken at the start, but issues were resolved within days and sometime hours of reporting them.
US based. Low ping is your friend, I suppose this could be a con if you're connecting from EU or Asia.
Cons:
Population. It would spike at times but usually it was a ghost town.
US based! The ping is great but they're very much within Blizzard's reach if the company decided to create some problems.
Emerald Dream
Pros:
Very healthy population. This includes at the end game and leveling,
there isn't really a shortage of raid guilds. Not too difficult to find instances while you level, especially if you're playing a tank/healer or proactive about forming a group.
Instances / Raid encounters are properly scripted. People like to shit on feenix for their horrible scripting, and it was probably deserved with their other servers, but ED is pretty top notch. The most non-classic feel you get from them is that they are buffed up a bit to compensate for 1.12 talents.
Cons:
EU based. If you're connecting from the US expect 150-225ms depending on your location and connection.
Content release / fixes seem to come in waves, and it has been quite a while since their last major patch. (Sound familiar?! ;D)
Like with most popular EMUs people love to DDoS the shit out of it. The problem is dealt with when it arises though, haven't had any issues since the server upgrades ~3 weeks ago. When script kiddies do connect though it can last anywhere from an hour to a few days (worst case from my experience)