According to the description at the developer's web site, iBird Pro contains information on 926 species of birds found in North America and Hawaii. The point of the regional U.S. guides is to offer a subset of the birds contained in the iBird Pro app for a lower price by only covering birds that appear in specific regions of the country, so individual regions might typically have 500-700 bird species. If you only want to identify birds in your geographic region, and nowhere else, this might be all that you need or want from the app. On the other hand, if you want coverage of at least 3 regions in the U.S. or Canada, it's cheaper to get the iBird Pro app.
One consequence of this is that the menu option to search by State/Month is only found in iBird Pro for North America, and not in the regional apps. Again, this is according to the description of "How to Search" given on the developer's web site page:
You can probably write to the developer at firstname.lastname@example.org if you have specific questions. My understanding is that if you get the iBird Pro Guide to Birds (for North America and Hawaii) along with iBIrd United Kingdom and Ireland you'll have complete coverage of all the bird species supported by their apps.
This link on the iBird web site has a table that shows the details of all the products they offer for birding http://ibird.com/compare/compare-ibird-version/
Esther is correct that the regional versions (West, South, etc) cover a number of states or provinces of the USA and Canada. For that reason they cost less than iBird Pro which covers all states and provinces of North America including birds endemic to Hawaii. Also Pro has more search attributes (38 total) vs about 15 for all the other apps. One of those Pro offers is the state month attribute which can come in handy.
I think if it was me I would peck the bullet and buy Pro so I had all the features. But if was not a real serious birder and didn't plan on traveling outside my region, or didn't want to know about birds outside my region, I would save $10 and get a regional version.
However when I think about all the money I have spent on bird related printed books, each of which cost $20 to $30, iBird Pro looks like a tremendous bargain.