I have both the MX900 and the BT500 as well. I ditto everything SpineDOc said.
Neither of them have any transmission delays.
The MX900 has a cradle, although regular AA batteries can be placed in them. The main problem with the MX900 is that it cannot be shut-off like the BT500 can. Although the MX900 is not really a travel mouse, I do not always want to place the mouse in the cradle all the time. Sometimes I see it flashing all night. At other times, the lengthy non-use causes it to freeze, and I have to reset by removing one of the batteries.
This, however, sometimes occurs with the BT500 when I am using it.
Can’t speak for the microsoft, but it looks pretty cool and looks comfortable.