Running Ubuntu on MacBook Pro 2015 — run natively (dual boot) or run in virtual machine for longer battery life?
Booting Ubuntu on a dual boot partition has a similar battery life to OS X, perhaps slightly shorter, due to less efficient optimization for the MacBook Pro's hardware.

In a virtual machine, despite lots of "clever" optimizations and CPU hardware virtualization features, there are two complete operating systems, with two kernels, and two display managers* running. There is no way around the fact that these require significantly more processing power, thus, sucking up more battery life.

If you want longer battery life in Ubuntu, try disabling the eye candy. (This, of course, is assuming the virtual machine is not running headless.)