Sorry, I should have clarified that Cavour is an excellent alternative for "nerds" like us here who knows their fit. It's not at all an alternative for regular folks who just buys their first suit - they need to walk in to the store and get fitted.Cavour is a great alternative only if RTW fits you well. Cavour doesn't really focus on MTM even in their physical store, and it tends to be marketed as a service they "can do, if you really want to". The MTM service is not made by the same people who make their RTW-line.
They do MTM through Atelier Munro and Orazio Luciano, however, both are much more expensive than SS, and SS is better quality than AM. OL is more than 5 times as expensive as SuitSupply and obviously involve a lot more handwork. MTM through SS is definitely superior to Atelier Munro, and you get much better value than MTM through Orazio Luciano per dollar/euro spent.
I'm curious to know exactly how Cavour's own RTW-line has better craftsmanship and quality, and remember that they are both made in China and use many of the same Biella fabrics. I think Cavour has better looking cuts on RTW, although very limited; i.e. notch lapels, spalla camicia and patch pockets on nearly all suits and jackets with a few exceptions with flap pockets.
Cavour's quality is much higher - they have a lot of handwork like all button holes are hand made, double pick stitching is handmade, collar and lining attachments, while Suitsupply's items are completely machine made with the rare exception of handmade Milaneese buttonholes. Also all Cavour's jackets are full canvassed, while Suitsupply barely has any