Prefab sealed boxes are usually fine, assuming they are built well and have the volume needed for your speakers. Internal volume is really the only applicable spec that varies performance in a sealed box (again assuming its built adequately). Ported boxes have a bit more complexity, as they also have a tuning frequency to add to the equation. But even then, assuming the box is built adequately, and the tuning/internal volume is right, prefab would be fine. Bandpass boxes are (relatively) much more complex, and slight variances from one sub brand/model to another can alter the required specs of a bandpass box very much, thus prefab bandpass boxes are almost always a failure.
Custom boxes, assuming the builder is competent, ensure the volume and tuning (if applicable) are correct. Many times custom enclosures also come with expereince, from the builder, that adds his/her personal touches to the box that can help the final performance of the product (such as cone loading on each sub in a shared-chamber ported setup, bracing, smoothing of the ports, etc). Custom boxes also ensure the box is built to fit your vehicle's dimensions well, which means improved aesthetics, ease of install, useable space after the install is complete, etc.
Pro-rabbit is a very knowledgeable builder and Id recommend using him if your budget and situation allow it.
