Well while not an expert I'm facing the same question, so here is the current state of my thoughts. I recently got an AN/DP c-card (actually I'm still waiting for the actual c-card) from a reputable TDI shop in Hurghada (excellent experience btw). I'm quite sold to the advantages of helium but I'm willing to dive where I live (UK) and where my family lives (France), countries where air dives to 60m officially and sometimes more (less officially) are still pretty much frequent. Helium for 45m dives is almost unheard of there, at least outside of caves, so I choose the DP over the Helitrox course. I had a few dives around the 40m mark and felt the narcosis was very manageable, the DP gives more time underwater but the depth increment is reasonable (40 to 45m). The keypoint here is : in the conditions of my mainly tropical dives.
At this point I think more about diving in the forties in conditions different from my training (ie. drysuit in england for instance) to get more experience, but later I think I'll pass on the extended range and go directly to normoxic trimix, simply because tek diving is a dangerous sport and I don't feel like adding more danger is needed. Deep air in ideal conditions is still an open question for me (if Helium unavailable for instance) but if I do it will probably be after my normo and with somebody used to it in a kind of apprenticeship. ie. I prefer to become familiar with this kind of diveplan and contingencies before, then, maybe, do it on an slightly impaired brain, if I feel my comfort zone is large enough to handle it.
Critics and suggestions on this plan from natural born fishes and more experienced divers are of course welcome !
