Having both, and just having completed a home renovation using both, along with the framing nailers, I'd venture a guess that the finish nailer might be more useful. It will handle a fairly large finish nail, as well as a fairly small one, both suitable for most household projects. The brad nailer might be a better choice if you're intentions are to build a lot of furniture.

There are differnet types of finish nailers. Finishing nails are nails with small heads that can easily be hidden with putty. Brad nailers are even slimmer nails and are mainly for light weight applications like building things like cabinets and such. The main difference between a brad nailer and finish nailer is the length of the nails it can take, anything 2" and longer is considered a finish nail, if it takes 1 1/4" or less it is considered a brad nailer. There are also some finish nailers that take larger diameter nails and up to 2 1/2" in length. Most are 15 and 16 guage in diameter, brad nails are much less in size. I happen to have all three types, the one that gets the most uses is the 2" 16 guage straight stick nailer by Porter Cable. the other onw I have is the angle stick 15 guage by Bostitch which I use for installing hardwood flooring.