The magazine angle does not affect the way a nail appears from the nailer. Its purpose is just to save space. Average nailers that launch brands and staples are designed with a zero-degree magazine angle; however, some framing nails are flat, which requires a more extended magazine angle.
A 28-degree nailer is more compact and has clipped heads. In contrast, the 21-degree nailer is not as smaller and has full-heads. Usually, for a heavy air nailer, a 21-degree magazine angle is ideal as nails with full heads are needed nails.
Besides, your gun does not need as much power for installing interior trim, so a 28-degree nailer would be more convenient in that case. The 28-degree nailer saves more space and is perfect for tight corners compared to a 21-degree nailer.