There are pros and cons in the debate AK or Marion without money consideration. But the most serious concern with AK is his health. Assuming talentwise they were a wash, the contract aspects will dictate that AK +Giricek for Marion + Banks with the other two players being cap fillers sounds relative fair.
Now, I prefer AK over Marion regarding skills .
1. AK will be able to score down low against most players smaller than 6'7, so teams can't hide the likes of Harris, Terry, Parker on him on defense and harass Nash with a big athlectic 2/3 guy.
2. He is a much better position defender and help defender, due to better fundamental. The only area Marion has over him is the ability to defend PGs with some success. But outside Parker, this is not really needed. And with Spurs, we could always hide Nash on Bowen and move Raja on Parker etc.
3. On offense in general, AK has a better release and attacks the basket much better. Well, Marion is probably the best finisher at fastbreak, but he is gone anyway you look at it even if not for AK. Within the league, AK's fastbreak finishing is still among the better 3/4s. I agree, like Marion, AK is also a fair-weather player and can't be relied on under pressure. But we have Amare and Nash for that anyway, and Hill is a vet good at that too. As to the total offensive production, once AK plays in this system, I believe he will be almost as productive on paper as Marion, with say 17pt in 35 min.