Well, they're both unofficial sequels, I don't know too much about Chrono Crisis, but I believe it follows a different party than Chrono Trigger. It's being made in RPG Maker, and it looks pretty clean.
Here's a Youtube video, if you're interested.
Crimson Echoes, on the other hand, is a ROM hack. It's being developed by some of the fellas here and, from what I've played, it seems like it's going to be a midquel, starring the original CT party. One of the main points it looks like it's going to explain is the Fall of Guardia.