Yoast's SEO Plugin wins hands down in my opinion as well.
There's nothing wrong with AllinOneSEO - it offers the basic ability to manually enter meta data for each page/post as necessary, and to create a default meta-data template for those pages you're not going to customize by hand.
Where Yoast's plugin shines is that it will actually help coach you through WHAT you should be entering in those different meta fields, and will help you do an essential analysis of every new piece of content you write in order help you improve it before it's even published. The visual presentation of what your SERP snippet will look like is also invaluable.
Many of these aspects are second-nature for experienced SEOs but provide helpful coaching for those not focusing on it full time. Even though I do SEO extensively, I still find having the tools/prompts right in front of me on every post to very beneficial.
In addition, Yoast's plugin includes several important additional SEO-friendly features (like breadcrumbs,etc) that otherwise require additional plugins. WordPress ALWAYS runs better the fewer plugins you have installed.
Paul
