[Darren27]

Equiv air depth?.  As I see it for normal rec diving, which it sounds like your into, you dont need it.  Most people, do not dive on tables so using either the BSAC nitrox table or calculating the EAD and then using an air table is not really an issue.  I suspect most people just set the computer then get in the water.  If that is the diving you do then the BSAC course would work out the cheapest.  The down side is you may have to wait till a course is run locally to you, but combined nitrox courses (the only one to go for really) is popular and are quite frequent.  If you dont want to wait then do a course while on holiday, the choice is yours.

The TDI adv nitrox course may allow you to use upto 100% but unless you have done the TDI deco procedures course you cant do any deco, so whats the big deal about using 100%? As to the BSAC courses not being suitable for going on to trimix, for my dealings with instructors, the equiv TDI nitrox course course is the same.  You would have to do the TDI deco procedures course after both before going on to the trimix course.  EAD is a very easy concept and can be mastered in ten minutes with an instructor.

HTH