I have Sonar X1 and X2 now. I started a song long ago in MC6 and have now transferred it to X2. I used a sound effect loop in MC6 that I can't find in X2. Does MC6 have a few things that X2 doesn't? Should I install MC6 on my new pc?

Possibly, and it won't hurt.

I installed MC6. The sounds I was looking for were in the song construction kit. Now I can't remember if it was in X1 or X2 or not? Hahaha. Oh well it's in there now.

It is possible MC6 has some different sounds in it so installing MC6 will not hurt. You should be able to access all the sounds from any of the DAW's once you install the sounds.... just watch the installation destinations so you can remember where it went.  Then add that path to access the sounds from any DAW. 

Better yet, put all the sounds in one folder location if they are not currently together.