Hi @gspencer,
Thank you for your question about choosing nail guns for your projects.
A framing nailer is predominantly used for rough framing, where the timber is likely to be clad with another, finer material that will hide the roughness. They fire thicker, more robust nails, which are better for creating solid structures like stud walls and flooring systems. Being a larger nail, they displace more timber when they are fired, and they leave a much larger entry hole, which is why they can't be used for finish work and are more prone to splitting timber.
A brad nailer is used for finishing work. They fire thinner nails, which are driven below the surface of the timber before being covered with a suitable filler and sealed. As the nails are thinner, less timber is being displaced, and the entry hole is smaller, meaning they are less likely to split the timber, and the holes they create are much easier to conceal, which is much better for finish materials.
As a very basic rule of thumb, a framing nailer is used for structures, and finish nailers are used for attaching cladding or finish materials to those structures.
For your deck framing work, I would be much more inclined to recommend a drill and driver. You really wouldn't get that much benefit out of a framing nailer, as decks are almost exclusively built with screws.
Of the two, a finish nailer would be a better investment, as it can do something a drill and driver can't: it can fire nails that are almost imperceptible to the eye, meaning they can be covered with filler without being obvious. A finish nailer like this Ryobi 18V ONE+ 16GA Brad Nailer would be good for attaching the merbau pickets to your frame.
Let me know what you think and if you have further questions, don't hesitate to ask.
Jacob
