: Having one website pros: Easier site maintenance since everything is all in one code base Allows your users to find all of your services easily since they are presented together You can create

Having one website pros:

Easier site maintenance since everything is all in one code base
Allows your users to find all of your services easily since they are presented together
You can create a natural hierarchy in your website which helps promote the top level pages
Cross-linking your inner pages is easier
Statistics can be all in one place
Cross-selling is easier since the user never has to leave your site

Having one website cons:

Only one domain name to get keywords in
If for some reason this site gets banned then all of your content is down

Having multiple website pros:

Each domain name can be keyword rich for the product or service
Each website is focused on one topic which offers less opportunity for confusion or distraction
If one site is banned the others will be unaffected

Having multiple website cons:

You have five code bases to manage. Even if they are virtually the same keeping them synchronized can be a pain in the butt.
If you want to promote your other websites you run the risk of duplicate content if the text is the same across websites

Non-factors:

The content isn't going to change
The page titles won't change (except for possibly the website tag line at the end of the title)
Some will say that multiple domains is better because of "trust" or other link benefits. In reality it doesn't change much.

Misc:

If you do go with one website make sure you do 301 redirects from the old URLs to the new URLs. That way both users and search engines know where the content has moved to. This is especially important for SEO since most of your link juice is transferred with 301 redirects. If you don't do the 301 redirects you will essentially be starting from scratch.
If pages will be going away after the merger redirect the user (with a 410 HTTP response) to a page that lets them know the page no longer exists and where they may find similar or other useful content on the new website