It's really up to you when you apply. Some people apply as soon as they can (six months ahead of the wedding) and some wait until right before they leave the US to apply.
Applying in person, in my opinion, is easiest. I guess it's easier on the nerves -- we applied for my spousal visa in person and I felt better about being right in front of the person when we applied. I was far too nervous to wait by the mailbox for a response!
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Still, again, it's up to you. If you're resonably close to a consulate though, (NYC, LA, or Chicago), I'd just apply in person.