Face it, a simple migration saves you lots of work. Apple's Migration Assistant transfers information when you set up your new Mac, letting you retain all your preferences and passwords on your new system. You pick up one one machine where you left off on another.
So why do so many of us here at TUAW manually set up our computers? It's all about cruft, that stuff that builds up over time. Cruft includes apps you no longer use, preferences you don't really remember setting, bits and pieces scattered around your system that accumulate with use.
We love unboxing, smelling that captured Shenzhen air, and setting up from scratch instead of bringing all our old mistakes and history along with us. And quite a number of us do this whenever we upgrade.
Sure, it's a much harder process. I have a migration log that I use each time I set up a Mac. It takes at least a half day of work, waiting for applications to transfer, re-entering serial numbers, adjusting the way my scrollbars work ("Show scrollbars: always, Mouse > Scroll direction, natural: disable), and tweaking my hosts file (someonewhocares.org). And that's just a taste of the customization I do.
Manually setting up your system transforms an automatic act into a deliberate one. It provides an excellent opportunity to question historical choices and evaluate how you want your system to behave. It's a natural break point, where you can reconsider how you've designed key system elements and where you can try out new ones fresh.
It gives you a sense that your new computer really is new, complete with "new car" smell. You wouldn't want to move over all the crumb-infested juicebox-stained seats from your old minivan to your new Porsche, so why would you want to do the same for your Mac?
Agreed, that's not a fair metaphor -- but it gives a sense of the ownership some of us want to feel over our new system. There's nothing there that we didn't deliberately put there. Sure, that sense of control quickly fades away as we start building up caches and attachment folders, temp files and so forth. But at least for a while, our system is fresh. And that's something worth considering the next time you upgrade.
What about you? Are you a migrator or a manual installer? Place a vote in this poll and then join in the comments with your thoughts.