Okay, here are my thoughts on Mu vs Lambda.
Close range: Mu > Lambda
If you look at this video: http://www.youtube.com/watch?v=PNyFo3e6MS8 , the Mu was using her lasers to get in close to Lambda. Unlike Lambda, Mu can't beat an opponent from far away. My guess is that Mu's job is to use her lasers as a cover to get in. Keep in mind that Lambda has a DP, but if she can't use it, I'd rush in with Mu.
Mid range: Mu < Lambda
Whatever Mu can do, Lambda can do better. Not to mention Lambda has 5C and 2147D~C.
Far range: Mu < Lambda
Lambda doesn't need to set things up. She has 214D~C, and 236D~C, as well. Mu has to be very careful when summoning her Steins, so Mu should do it when Lambda is downed. If Mu doesn't, then Lambda could punish Mu's recovery phase with 5D or 4D. Even if it's not a counter hit, it forces Mu to block again.
To me, it seems like this is a game of "Who's First." If Mu can get Lambda in a corner and effectively set up Steins in a safe manner, then Mu has a chance. if not, Lambda can just zone Mu and force her to block.
If anyone disagrees, then we can discuss and come up with a collaborative idea on the match-up.