Hi Belinda
Tough question! From my personal experience, i have found one month that the OPKS and Ferning tests to match up in prediciting O (of course with temping and CM checks). Then another just OKS and then more towards ferning being closer to the O date.

Some girls find the Ferning throughout their cycle and some never get anything more than pebbles. I find it hard as I chew up a lot and i found this made a difference to the results i got.

I have used Ovunow and find that they come up with the lines for me and finally I got a line as dark as the control one eventually. I have longish cycles now and O anywhere from day 16 to 24. And the fact that you are supposed to start testing before that date, so even 10 days of Okps if im good and only do it once is about $5-$6 with that brand, compared to about $18 - $40. Thankfully they work for me, although i am starting to buy another brand to use two to double check but am getting used to what my cycles look like and when im going to O. Everyone is diferent with their choice. And its hard to reduce the fluid intake in winter when theres nothing better than a hot cuppa to warm you up.