Welcome to the forum.
Does MC 2 support VST instruments and effects? Using VST instruments can expand your library of sound immensely.
Then there is the operating system problem, since many of the older programs won't work properly on Win 7 or 8... possibly not in Vista, too. We may all be forced to evolve when our XP computers finally die.
You will still be able to use the mouse to enter data in MIDI tracks. Which keyboard are you referring to? A MIDI keyboard or the computer keyboard?
You will still be able to use a MIDI keyboard with MC 6.
If you use the Staff view to write much of your music and use the keyboard shortcuts in the staff view like I do, you'll be greatly dissappointed that they have been removed.
But it's still an inexpensive program with a lot of bang for the buck. The VST support will be especially valuable to someone looking for more and better sounds. If you can afford it, it may be worthwhile to get it anyway, but keep MC 2 loaded in case there are things you like better about it.