It depends on what you are doing. They both have advantages and disadvantages, just in different places. DPD can distinguish between FC and CC, which OTO can't. On the other hand DPD will show zero chlorine when the FC level is actually very high. DPD is noticeably better at distinguishing between FC levels around 3 to 5.
The FAS-DPD test is much better than either of those in nearly every way.
I find the OTO test kit included in the TF100 kit to be handy for quick ball park chlorine level checks, but it is a BIG ballpark and it is also handy as a double check against the FAS-DPD, I have used tests that were DPD in the past, they are somewhere in the middle, a little easier than doing the FAS-DPD, but have the color matching problems of OTO.
OTO - quick easy 1 step ballpark total chlorine number
DPD - mid level test but tells you both FC and CC (at least in a ballpark)
FAS-DPD little more work than DPD, but tells you to the .5 PPM level for both FC and CC and best of all NO COLOR MATCHING just pink or clear
I went to my local pool store (Pool and Spa Depot) and asked to see if they maybe carried the FAS-DPD test and they didn't have any idea what I was talking about. I tried to explain to them what it was and the man behind the counter told me, "We don't get that technical." I was taken aback and decided that there was no way I was going back in that store again....ugh. I am going to have to eventually just get the FAS-DPD test kit so I can test accordingly. For the time being I will continue to use the OTO for daily tests and the DPD maybe every other day to TRY to verify.
You would also need R-003 for the combined chlorine part of the test, a graduated cylinder to get the correct amount of water and the little scoop for the powder. To me is seems easier to order the K-1515 kit as you get a box to keep it in, etc. Note if you use non-chlorine shock like HTH Shock n Swim you want to get K-1518
You can either get the Taylor K-1515 FAS-DPD chlorine test kit here or you can get the FAS-DPD chlorine test kit from TFTestkits here with the latter kit having 2.67 times the volume of reagents so is less expensive per test.