Okie dokie. Nailers 101...
Trim nailers are usually about 16 gauge. They'll typically shoot nails ranging from somewhere around 1" to 2-1/2" long. There are angled finish nailers and straight finish nailers. Either one will work fine, and it is a matter of preference. A trim nailer is what is needed to install base, casing, crown, chair rail, and general trimming around a house.
Brad nailers are usually about 18 gauge, which is thinner than a trim gun nail. Brad nailers are great for smaller trim applications that involve nailing wood to other wood...Small mouldings, cabinetry, drawers, etc. My brad nailer shoots brads 5/8" to 1-1/4" I think. I use it a lot more, but I do a lot of woodworking. It is also nice for installing base shoe, as the thinner brad is less likely to split the wood.
If I could only have one gun, it would be a finish nailer, not a brad nailer.
You can find some great deals. Last year I bought a Ridgid combo kit at Home Depot for $118. It included a brad nailer, a trim nailer, and a 9.6volt driver. That's two guns for the price of one, and a free driver to boot. Honestly, both nailers have performed flawlessly under a lot of use. I am really happy with them. I had Bostitch guns before, and they were more of a problem than an asset. I've had good luck with Senco and Paslode guns. Not a fan of the Porter Cable nailers, although I generally like their tools.