Is this SP 2013 or SP 2010 ?
Generally, it is cleaner to give each department there own site collection.
But, there is one thing to keep in mind when splitting your departments across different site collections vs keeping them in one. Cross-SPSite query'ing is usually a problem. Everything inside an SPSite is more or less accessible from accross other Subsites, but once you need something from outside the SPSite, you will usually see that it is troublesome.
There are several options for showing data from one site inside another:
- Content Query Web Part
- A good tool for aggregating content
- Needs a custom XSL to look cleaner
- List View WebPart
- With some customization, you can view lists from a different site with it (export the built-in wp's through SPDesigner for example
- Same as SP2010
- Content Search Web Part
- Uses the search engine to aggregate items, cross-SPSite is possible
- Has some built-in templates but not as nice as a ListViewWebPart (like CQWP)
You should look up the software limits for Site Collections and figure out if you would need several site collections to accomodate your needs or not.