I run 34-24 for my local hilly rides and switch to a 22t cog for flatter areas. I think you first need to figure out what ratio you need, as the 2 you listed are pretty different. 32-20 works out to 1.6, and 36-24 is 1.5. The 36-24 will certainly be smoother and last longer, but it is also a lower ratio. I wouldn't worry about chainring clearance, 95% of the MTB'ers ride around with a 44 or 42 tooth big ring, and they get by just fine.
Larger cogs are harder to find though. Rennen and Endless both make aluminum cogs up to at least 24t. I have been running a Boone cog for a few years and love it, but I am scared what will happen when it wears out. I really don't like aluminum cogs. I am thinking of ordering a Ti cog form Homebrew components, but also not a cheap option.
Mark