Your power is only as great as your connection. Thus, your ability to use the Force, your strength in the Force, is entirely dependant on the amount of midichlorians. There is effectively no difference between midichlorians and the Force itself, the point is you can use the Force because of the measureable amount of midichlorian cells. Yoda says "strong am I in the Force," but in TPM Kenobi remarks that "even Yoda doesn't have a midichlorian count [as] high [as Anakin]", which is significant in that his power is framed in terms of a measureable scientific report.