Recording at 24-bit ultimately will give you lower noise floor in most cases, because you can raise or lower the gain on each track without introducing so much noise as you would at lower bit rates, which correlates to the overall quality of the end product.
Which two were you looking at? It all depends on what sort of inputs you need. The converter quality and the pre quality will all be the same. If you're doing drums and plan on doing it without a submix, the Tube, Studio, and 2626 will all work. If you plan on connecting many instrument level inputs, you'll need either the Project or the 2626.