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Worse from the customer perspective.

All their old apps now run probably 2 to 10 times slower, until new native binaries arrived. Compare that with the PPC->Intel transition, when old binary apps stayed about the same or maybe got a little faster.

Also, emulating Intel binary apps with the CPU pegged for twice as long nullifies any energy savings, so the battery life in practice gets worse. Even if it is 20% for native binary apps, consumers are going to see "new computer, worse battery life, Og smash!" during the transition.

You're not going to see a 20% gain in battery life for native binaries, though. The only way you'd see that is if the ARM CPU used no energy at all. Extraordinary claims, evidence, and all that.

Is there really any reason to think that an Intel and ARM chip of comparable performance will have significantly different package sizes? Intel has always been the leader in process technology, so it would surprise me if the ARM die were any smaller.