You'll get differing opinions on this, but I would think for your purposes one big library might be a better idea. Much of the useful stuff in Aperture is stored in each library. They are sort of unique databases of some complexity, and designed so that separate libraries are only needed when you need different settings. Generally I feel that unless there is a really good reason for it, no need to have multiple libraries, especially if you want some commonality between all the images. You have so many organizational tools within Aperture that there isn't any reason for separate libraries unless you need to export them, move a subset somewhere, etc. Even when I needed to do that I still kept one big master library anyway.
And it can be easier, if you do need separate libraries, to start with one big one with all the settings, keyword lists, and other stuff you need, and then exporting subsets of it to new libraries. If need be you can then delete all the stuff you just exported.
One big library is obviously a way to keep your preferences global, in effect. And not only preferences, but presets, keywords, all kinds of stuff. It can get really confusing if you are trying to use different combinations of albums and folders, or hierarchical keywords, in several libraries. There aren't ways to sync all that stuff easily. You can use folders and albums and keywords to achieve much of the same organization. OTOH, you can export and then import keyword lists from library to library. But as soon as you start using them and say change a hierarchy then you have to do it back in the other direction.
I can certainly see using a different library for say personal photos vs work photos since there are issues of access and ownership.
And a tip about keywords. You didn't have to deal with this in iPhoto, but consider the power of hiearchical keywords in Aperture. They can supply a LOT of organizational capacity to your images, so that even with many more images than you have finding the stuff you need is really easy. And the keywords are part of the IPTC structure, which means that they can (and should IMHO) be embedded in the photos, which makes them portable. So if a photo gets moved to another Aperture library, even on another computer, it carries some of it's organizational structure with it to it's new home. The hiearchies are sort of unique to Aperture but can be imported into other programs as well. And outside of Aperture you can use Spotlight to find photo keywords right in the Finder. Add in metadata presets you can use on import, for example, and you are gonna have some amazing tools to play with. Since you already have invested a lot in keywording, massaging those into hiearachies might give you enough organizational structure you won't need separate libraries.
Enjoy,
Rob
