Moving WordPress Blog to a New URL

I have a wordpress blog that I’ve been running for a while. It gets good traffic and has good placement on the search engines. I need to move it to a new URL, and I’m not sure what the best way to do that is. I’d obviously like to lose as little traffic as possible. As I see it there are two options:

1. Register and map the new domain to the existing blog, make it the primary domain, and keep the old url as a secondary domain. According to the FAQ information, it sounds like this would redirect all the old urls/search engine results to the new blog pages.
2. Buy the “Site Redirect” upgrade.

Obviously #2 seems the more “official” option, but I can’t figure out what it would accomplish that #1 would not. And it would be more expensive, since I’d still have to buy and map the new URL. Anyone know what the difference between these two approaches would be?