The ECM8000's are not good for ignoring off-axis sound. They'll pick up pretty much anything in the room. As I understand it, the MXL's are MUCH more directional.
The difference here is then going to be the overall effect. You'll get a lot more room sound with the ECM's. (depending on how far they are placed from the source, of course...) This may be good or bad. The MXL's will give you better off-axis rejection, making separation a little easier, but less room sound.
My budget only allowed for the ECM's, but I've been happy with them.
Chris