opk- do you go by the first positive or by the darkest line?

Donna
Hi all,
Do you guys (rather girls) think I should go by my first positive on a digital opk test or by the darkest line on a cheapie? 
The digital was positive 2 days ago but the line was not as dark as it is today on a cheapie (actually, it is now darker than than the test line)?
Thanks in advance