Hi Owen,
Happy to answer this for you! For one, while both utilities are used for switches, that doesn't inherently make them the same. Both operate completely different in terms of method, but also in presentation. M-Case is definitely more suitable for more informal settings, as you are just using the card case for storing the card or small item temporarily, while the Unbelievalope is suitable for both situations, leaning more towards the formal presentation and something you'd use in a parlour show, for example.
The Unbelievalope also has the full card in view the entire time, where the M-Case you can partially see it. And the M-Case does not allow for multiple outs, while the Unbelievalope does (13 outs!) with the new index system envelope.
Also, the very big difference that could make the choice between either that much more important is if you want the card signed. Unbelievalope makes the signed card prediction SO easy to perform, where the method with M-Case almost makes this impossible if you want it to look absolutely clean.
Both are switching devices but in totally different ways, so it's up to you which one suits your performance style and the type of effect you want to perform in the end.