It is said that a moderate smoking habit reduces oxygen uptake rate by about 5% on average. Using cannabis is likely to affect your respiration rate by less. So this is pretty small change for most athletes. It will only be in intense competitions of cardiovascular fitness, like endurance sports, where this is likely to be a really significant factor. FYI, Gianluca Vialli, the Italian footballer, won Serie A, the European Cup and the Italian Cup with Juventus, and was a moderate smoker (of cigarettes) the whole time.
I don't know what the answer to this question is. I suspect that smoking is less damaging of crude physical functions in the short run, if only because heavy drinking impairs your liver function quite significantly for a couple of days, and you need that bad boy to control the dispersion of nutrients through to your body. That is essential for allowing muscle development and recovery, and so will reduce the value of your training. (How much I don't know.) Cannabis doesn't have these kinds of effects, that I know of, so you ought to be better off with that.
Personally I found that after drinking heavily my balance was not good (odd that, I don't know why), and after smoking heavily I was simply not motivated.