I've used both and they each have they're pros and cons. LDD looks nicer (lags more) and the bricks actually attach. MLcad has a much larger database and has more control but you have a stupidly hard time using snot because the pieces wont auto alighn (or auto collide)
Have you had LDD delete bricks when you open them five minutes later
because it says they won't attached ?
I think the fact that Mlcad won't auto align is what makes it so flexible.
you've got 3 grades of control for the alignment grid and you can fine
tune these in setup. I went and changed the fine and medium grid
rotations to 1 degree and 22.5 degrees
I've never got around to installing MLCad though
if your happy with LDD don't, from speaking to people in other groups
even with help it is so different and they get confused.
if you decide to you will want to update the LDraw.xml so that your
exports work better.
you will find it here and instructions on how to do it.