AtTiny85 128khz clock and power consumption
If you select the internal 128kHz oscillator as the clock source, the 8MHz internal oscillator should be disabled, at least according to Atmel.  

Assuming the 128kHz oscillator itself (not the core running off the clock) uses less power than the 8Mhz oscillator (a pretty safe bet), then the chip should use less power when running off the 128Khz oscillator than it does running off the 8MHz oscillator divided down to 128kHz by the prescaler.

But of course all this is conjecture! If the answer is important, then this is not a hard thing to test. Program a chip with an infinite loop and then try both fuse settings and see how much power it uses in each config. If you don't have a sensitive enough current meter, you can try running the chip off a capacitor charged to a known voltage and then measure how long it takes to drop to another known voltage in each case.