I'm still totally undecided, but I think the best option for now is to plan/research for both routes, i.e. a route from Turkey > 'Stans > China, and another via Iran > Pakistan > India. Now at least I know that both are possible.
Thanks for all the information!

#9. I agree Turkey is fantastic. I cycled a bit of it a few years ago, and definitely would go back for more. My thinking regarding going north would be to get a taste of even more countries/cultures.
Going through turkey MIGHT make it a bit easier to sort out visa's for countries further on though. ie wouldn't have to get them from home and worry about timetables as much. Not sure on that one though.

I heard about people going Turkey-Russia by ferry and then on straight through Siberia to China.
You would have to investigate cycling in Russia in the middle of nowhere for this route of course - meaning taking along food supplies and water/filtering systems. (Or cheating and taking the Transsib.)