CKN are stronger than LB before the 4th promotion, after the 4th promotion LB becomes stronger since they don't carry over the attack penalty of the CKN and because CKN with logistics is wasted since there aren't enough movement points for 3 attacks. So in a standard game with a standard CB rush, you should generally have plenty of experience on your CBs so when you turn them to LB/CKN you should be at or very near the 4th promotion, making LB the superior choice most of the times. With that said, you need to look at the rest of what China and England bring to the table, not just their unique ranged unit.