Return to blockland is the old one Blockland is still getting updates but Is just That I'm indecise.
Also I like more blockland but Minecraft idk because it lags on my browser but what I tried to play it's good so I don't know all the things you can do.
You can get minecraft on a client which runs much faster than the browser version, blockland may be more intricate and moddable, but minecraft is still in its alpha version and who knows what Notch could come up with during the time till release.