I just did some math, and the CI offering from Microsoft (for open source projects) is ~8x more build minutes (60 x 30 x 24 x 10 = 432,000 vs 50,000); and they support macOS/Windows builds.
Their open source offering is pretty new, Sept 10.
From playing around with it, they've got some nice features (aside from 10 concurrent builds vs 5 on Travis) like a way to import build templates if you've got multiple repositories that share a build step.