My personal experience is that they are all consistently, consistent, until you start comparing them with other brands, then they all turn consistently inconsistent which can freak anyone out. Keep in mind, the differences are most likely nothing that is so far off that you are going to trash your tank if you choose one over the other (assuming all outside variables are the same - time taken, lighting, feeding, temperature... heck, the device you are taking the water from and even how it was cleaned can make a difference).
It's kind of like having a temp probe sitting next to another company's temp probe and you are a couple of a tenths (hopefully nothing more!) off of one another. You could drive yourself insane trying to make sense of it!
I will put my two cents in on strip testing. This is the one area I have gotten completely different results using same strips from same company and even out of same box... at the same time! But they are great for those that like to dip, walk out the door and know they are not in the process of melting their little nemo. JMHO
Oh yeah, pay attention to how long you hold on to these test kits. Outside conditions may play a factor, but I have had different results in using old liquid drop tests when compared to one purchased the day I got a questionable reading from my older kit.
Final answer - Personally I would stay away from strips and select a good quality liquid test kit (I personally use API and my fish don't complain). Just stick with whatever brand you choose and you will find greater consistency then bouncing all over the board. These company's spend tons and tons of money on research. They all now realize we are no slouches either - much better informed than we were even 10 years ago.
Now, if things start to go haywire... test... then test again, then test again with a new test kit (still same company is what I've done) and only then would I consider checking with another manufacturer. In 25 years (thank goodness) I personally haven't had to cross that road yet. Others will tell you differently, and I'm sure there are even others using something even better. This is just what has worked for me. Take it for what it's worth and keep reefing!