I completely agree with scubajcf that good materials are enormously important. As an instructor, you want your students to have multiple means of gaining an understanding of the topics you will be covering. Good print materials, good videos, and good instruction all work together to provide these multiple means.
I am PADI affiliated, but at one point had a brief association with SDI/TDI. I have copies of some SDI manuals and nearly all TDI manuals. I have gone through them, and I can tell you that they are very readable. If going on to Tech training is a priority, TDI has been well regarded in this sphere this since before PADI jumped on the bandwagon with DSAT, and in my opinion (even though I have DSAT blender ratings), the dive training TDI provides is just a notch more demanding than that DSAT offers. One shop here in Phuket with both TDI and DSAT instructors sells both entry-level cert courses more or less simultaneously by adding an additional day to the DSAT course to "top up" for the TDI cert.
On the other hand, SDI was created after-the-fact to lead into TDI--sort of the opposite of what happened with PADI. (I suppose that once a diver gets used to the PADI system and is comfortable with it, s/he would tend to continue with PADI, so as a marketing strategy, it would make sense for TI (the SDI/TDI umbrella org) to create a recreational level so that their SDI students would remain with TI and go on to TDI tech training.)
Additionally, I did my intro tech training through IANTD, and had a very tough time understanding certain aspects of it, in large part due to the poor quality of the materials--both the manuals and the powerpoints. TDI materials are readable, at least. One thing is certain for me--if I go on to further tech training, it will not be through IANTD, partly because I don't want to be guessing what the materials are trying to communicate to me, and I'm the kind of person who learns by reading, not by listening to powerpoints.
Anyway, your instructor ratings are not like a citizenship--you are allowed to affiliate with more than one at a time, no problem. Pick one, and then cross over to gain the rating in the other one, if it makes good business sense to do so.