OEM MX5 diff case - cheap, lots about. Awkward shape as it's wide at the top. Used in several kits unmodified. It does have a Mazda-engineered failure mode in the RHS arm so it will snap in accident and prevent the PPF from puncturing the fuel tank/acting like a battering ram in an MX5.
Talon MX5 diff case - more expensive, but will be very nicely engineered (I'm very happy with the quality of the Talon parts I have). Much more compact, will give you more fitting options WRT chassis choice. Customisable; you can put whatever mounts you want on it to suit.
HTH
