They are not interchangeable, the ideal is both. A wavebox creates a wave type flow, which if you have ever been to the coast or a large lake, you know the result. You kick up some sand, it washes back and forth, hovering in the area you kicked it up and slowly but eventually falls essentially right back where it was. This type of flow is useful, it lifts and suspends detritus, gives a natural look, seems to encourage a feeding response and gets some flow to all areas of the tank. On its own though, it generally is not sufficient, nothing is really moved to the filters or overflows, it needs to be paired with directional current. You can accomplish a wave with a pair of 6105's and the right timing on a controller, but the same principles apply, the wavebox is just more energy efficient as it combines the pumps pulses with displacement of the box. As a starting point, I would go with the 6105's and use them in an Interval mode, when you get the bigger tank, it might make sense to add a wavebox. A nano wavebox would be sufficient for a 120, but not for a 210.
