Well, the price differences arise from a couple of features of convenience, performance, and specialization:
1. pressure - 3000 vs. 4500. Naturally a 4500 psi tank holds 50% more air and is priced upwards accordingly. This is convenient for sizing down on your tank for a lighter and more compact gun or supplying your playing style (need for more shots) and/or gas hogging gun (Shockers and Matrixes primarily) with the necessary amount to get you through games comfortably.
2. adjustability. Most guns can accept a fixed output pressure from a preset (800-900 psi) screw in tank, the simplest and cheapest form of nitrogen system. However, some by their design require lower pressure for operation and most can be optimized by setting a specific input pressure for working with the gun and its reg.