Moving wordpress: redirect best practices?

Hello Friends
I will merge 2 blogs located in subdomains. Both are the same blog but in different languages:

I'm going to merge them in (I'll use wpml to handle the translation) bit I'd like to know your opinion on how to handle the redirections.

The easy way is to send everything to the new domains. But I'd like to send each post to it's new location:

go to

This is because I want to keep the ranking of the blog.
Is there any tool to do such thing? And what do you think about it. Is it worth going trough this details?