I get confusing description from various sources. The unit itself shows 55w output power on both of them.

The descritpion on beocentral.com indicates 40W for ML MCL and then 2x30W for beolink passive. Physically they look exactly the same both have the IR inout, ML, PL input and the 2xDIN outputs. Ofcourse the "eye" was not supplied with ML MCL and came as a package with beolink passive. However, the 2007 price from becentral shows MLMCL at 390 pounds and 550 pounds for beolink passive - why is there so much difference

Am I missing some thing here...........

They are the same internally.  The price difference is only for the link eye and cable.  

  ... if you add an IR-Receiver, some mounting hardware, a 5m cable to connect the IR-Receiver 
to a ML/MCL-Converter and the Guide for the BeoLink Passive you'll get a "BeoLink Passive" for
approx EUR 30,- less ...

but - maybe your dealer will charge you an extra charge of EUR 29,99 for ordering these parts separately    

Are there any variations in software between the two units?

Matthew:

Are there any variations in software between the two units?

As far as I can remember, none. At least the recent SW 1.6 (B&O part/order# 8006128) can definitely
be used for BeoLink Passive as well as ML/MCL converter.

History:
SW 1.4 - Solves problem "no sound in linkroom" when unit being operated in low temperature environment 
               (in cold rooms during winter)
SW 1.5 - A.AUX is supported
SW 1.6 - ML Gateway (MLGW) compatible

Thanks Vienna.

If one requires an update, presumably it's a visit to the local dealer with an open wallet?

Matthew:

...presumably it's a visit to the local dealer with an open wallet?

SW-Chip is  +/-  EUR 45  +  costs of  labour  (15 - 30 min)