Ultimately, Xcloud's secret sauce is that they've already got the Xbox One back catalogue to tap into, plus continued support from developers (since I suspect that all Xbox One games moving forward and all Scarlett games will be "Xcloud ready"). Stadia, in comparison, is basically starting from scratch and reliant on developers making Linux ports.
The one advantage Stadia has is that they're going to be offering a free tier, which means that anyone (in a supported country) can buy a Stadia game without any further financial commitment (to Google). It doesn't appear that Microsoft is planning on matching that, but as Phil said, the precise business model is TBA.