You get what you pay for. Just take the pH test, for example. Many other inexpensive test kits use phenol red for the pH indicator dye but they don't have any chlorine neutralizers in them or they use sodium thiosulfate which itself affects the pH. The result is that the pH isn't accurate for a range of chlorine levels. That isn't true for the Taylor pH test (used in the K-2006 and the TF-100) because they use a proprietary blend of chlorine neutralizers that don't affect the pH much and handle an FC up to around 10 ppm or so.
Then there's the FAS-DPD chlorine test. It's more expensive than the DPD test (using a visual comparator), but far more accurate "counting the drops" and with a much larger range. Because it is the active chlorine level that prevents algae growth, it is very important that the FC and CYA tests be as accurate as they can. Furthermore, if one needs to use an elevated chlorine level to kill off algae or for any other reason, the FAS-DPD test can measure chlorine up to 50 ppm while the DPD will bleach out not able to read accurately above 10 ppm (best case; in some kits it stops reading well at 5 ppm). When doing an Overnight Chlorine Loss Test, one needs accuracy and when measuring Combined Chlorine (CC) one wants to know if it's really <= 0.5 ppm and can use FAS-DPD to test within 0.2 ppm if desired.
And don't even think about using test strips. Most don't even test for Calcium Hardness (CH) and instead test Total Hardness which is not relevant since it is calcium carbonate that protects plaster surfaces, not magnesium which is also counted in Total Hardness. The test strips are hard to read accurately and are dependent on technique. While some are better than others, it's not worth using them if there is a better drop-based test. See
this post for a comparison of some test strips against the Taylor K-2006 and TFTestKits TF-100.
As for the CYA test, the one in the TF-100 can measure down to 20 ppm and the
separate CYA test for $17 has enough reagent for 16 tests. The roughly $10
CYA test on Amazon has a 4 ounce bottle for only 8 tests (if they are 1/2 ounce per test) while the
8 oz bottle test on Amazon costs $19. But that's not all -- the test uses melamine to precipitate melamine cyanurate and then visually measure the CYA via turbidity (black dot test), BUT melamine cyanurate is soluble in water at around 20 ppm. In order to get an accurate CYA number, one must use a low pH buffer solution, which of course the Taylor has built into their melamine reagent. Does the Blue Devil reagent do this? I can't tell.
As for the amount of reagents in the TF-100, the indicator dyes are the most sensitive but if you store the kit in a cool place away from sunlight you should be able to keep the reagents for 2 years. Certainly all the acids/bases in the test will last forever. See
this post for more details.
If you just want to spend less money and don't want as many reagents, then the
Taylor K-2006 will be $50 instead of $68 for the TF-100, but you will be paying for extra reagent volumes for tests you don't do as often and will have less reagent for tests you do more often. See
Test Kits Compared. If you want to get the K-2006 that is still a fine kit and you can later get larger refills. This would also be a way of getting the K-2006 pH comparator which with 7.0,7.2,7.4,7.6,7.8,8.0 is different than the TF-100 which uses the K-1000 comparator with 6.8,7.2,7.5,7.8, 8.2.
