The strip version of them with the little pads on the strip are the cheapest and least accurate. I know the drip style where you add a few drops from a bottle into a small vile of water are usually pretty good. I've never used or read up on the digital ones, so I can't compare them.
Personally I use the strips because they are cheap, fast and super easy. These can be good for referencing your numbers, making sure everything is about where you want it. If you are having issues or are doing something to adjust your numbers, then I would suggest using a higher grade test.
just my two cents worth...