[Daniel101]

From my experience, there is very little actual difference between courses with different agencies. A far more important consideration is the quality and experience of the instructor you choose.

The DSAT course, IMHO, offers the best quality of learning materials and follows the PADI teaching system - that many divers are already familiar with. TDI and IANTD have less professionally produced materials, but have a wider variety of instructors qualified with them - meaning your choice of instructor is more critical. BSAC are relatively new to the technical teaching market and I've seen some bad stuff there (not to say that all BSAC tech instructors are bad though!!!). Another option is GUE. More expensive and very rigid...but the quality of instruction will be high and your 'core' skills will be extensively developed.
The structure of courses with different agencies does vary, but all essentially follow the same pattern. Firstly, you will learn to use higher nitrox mixes (>40%), followed by training in decompression skills. Lastly, you will put these two areas together and utilize rich nitrox for accelerated decompression. The initial stages typically use air as the leanest bottom gas - meaning that your max depth is limited by the PO2 of air....and tolerance for narcosis.

To venture deeper, you need to reduce FO2 in your bottom gas and raise your Equivalent Nitrogen Depth...and this will mean substitution of some of these gasses with Helium....a Trimix course.

I like the way that DSAT has 'done away' with the concept of breaking these areas into different courses....you simply do a Tech Deep (accelerated deco, using air as bottom gas) and then a Trimix course. IMHO, far more streamlined than doing an Advanced Nitrox, Deco Procedures and then Extended Range mix of courses. Also have a look around for the new SSI TXR course…….as the basic Tech course (all-in-one like the DSAT course) also includes Normoxic (Entry Level) Trimix training/certification.