I will explain to you which one to go for according to me.
AIO SEO lacks some additional capabilities that Platinum SEO has.
The configuration pages for Platinum and All-in-One are similar, but Platinum adds a lot of capabilities:
* Platinum SEO can automatically do 301 redirects for permalink changes
* Independently set noindex for author-based archives, as well as category, date and tag-based
* Set noindex for paginated comment displays (GREAT feature!)
* Set noindex on RSS and RSS comments feeds
* Set noindex for sub-pages
* Add noodp (Open Directory Project) and noydir (Yahoo! Directories) tags
* Add nofollow to category and/or date-based archive listings on pages and/or posts
* Add nofollow to tag page links
* Add nofollow to external links on front page
* Add nofollow to login and registration links
To be fair, there are also a few things that All-in-One offers that Platinum doesn't:
* Dynamically generated keywords for Posts page
* Specify a list of pages to exclude from optimization
* Capitalization of Category titles
Clearly, on a feature-for-feature basis, Platinum SEO outranks All-in-One. It even offers a tool to auto-import all your AIO settings once you've activated it.