Do I let Migration Assistant move everything it shows (user accounts, applications, ect.)?
There are NO apps on a windows computer that will work on a Mac. There is NO reason to import any user accounts. Why you have one on your Mac already.
IMHO Migration Assistant is a POS and Should Not Be used. Just copy over your personal Files.
If I don't use Migration Assistant (can't do setup assistant b/c this mac already used a bit) what do I use? A hard drive? Plus, I need some pictures and music from a PC (Adobe Photoshop).
That or file sharing over a network connection.
Lots of folks have done fine with Migration Assistant.
But if you want to do it manually, see the Manually Migrating section of http://support.apple.com/kb/HT2518
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