PH Testing

if it resolves down to .1 pH, it's useable. You don't need precision beyond that for most things. As far as which is most accurate. If you have some callibration liquid of various pH you can see how they do. Unless you have a blue labs callibrated meter or equivalent, it's going to vary a slight bit, but again, don't need lab precision on this. If you randomly sway +/- .2 pH from whatever optimal is, it could actiually be beneficial and rarely would that be an issue... maybe in hydro? your soil will buffer and correct pH too.

That being said certain nutrients will/can make your drops very hard to read.. Pens are more accurate in this regard.

IMO the most reliable is litmus paper...