The Career Average methodology looks to me to be much more valid given its based on at least some kind of empirical evidence of the value, in terms of starts, of drafting in a specific position. Not all starts are created equal, however. I can't find anything which supports the Johnson chart which I suspect is based on historical precedent.
Discussion of discounts based on a time value concept are pretty interesting. What is the value of one season of (projected) play and how is it measured (wins, revenue, cost)? Using the rookie wage scale to do a quick PV of future salaries/signing bonuses is the only way I can come up with off the top of my head to arrive at an apples to apples comparison, which has many glaring flaws.