That's why we didn't do this with machine types. If you don't care, you don't have to. If you care, ask for a particular CPU platform (or newer). Most people won't and probably shouldn't care, but Skylake is enough of a jump that we finally are giving people the explicit ability to choose.
Both clock-for-clock and even more strongly for applications that can take advantage of AVX-512, Skylake can be a lot faster than either Haswell/Broadwell. Like the jump from Sandybridge/Ivybridge to Haswell/Broadwell, the jump to Skylake is a big one. For now the best comparisons are going to come from independent sources like Anandtech and hopefully from customers running their actual code on Skylake and publishing the results.
Disclosure: I work on Google Cloud.