Just to get it out of the way, preordering is bad. I won't take the time to explain why but im sure there's 20+ comments in this post alone that would love to spell it out for anyone who doesn't get it yet. Now with that said if you like both games, as many people have already said, get both. There's is plenty of time between their releases to finish everything bungie could hope to provide it's fans. The way i see it a small percentage of people will blow through everything in ttk in the first 24 hours. Another slighlty larger portion will finish within the week, and then everyone who games regularly will be done in 1.5-3 weeks time. Finally, for anyone else who only plays occasionally i can see them needing up to a month and half max. After that if your still playing your either grinding the same missions over and over for that last exotic drop you "need", or you love to run crucible with your group. Either way if you've got the cash for both, don't hesitate, and the words of Shia Lebeouf: "[i]just do it[/i]". :P