kdx or rm ? What is the major difference

I currently have 2 bikes a 91 rm 125 and a 1987 490yz.

I am going to sell the 490 and pick up a 92-93 kdx, I think.
I only putt around and would like to play on a mx track and do trails and very mild jumps.

I was wondering about the RM. It is a lot of fun but what are the differences in power between 125rm and a 220or 250kdx. What are the advantages of one over the other?

Thank you,

kingriz