A is one thing, B is another
Definition 1 of 1
Pro Tip 1/3Contrast Emphasis
Emphasizes a significant difference between two aspects or ideas for clarity. 
Understanding the instructions is one thing, actually applying them is another.
Pro Tip 2/3Subtle Judgment
Can subtly suggest one aspect is better or more important than another. 
Creating art is one thing, but appreciating its value is another.
Pro Tip 3/3Conversation Shifter
Use it to change the direction of a conversation or introduce a new idea. 
I know you're busy is one thing, taking care of your health is another.