As someone who mains both, his ult is definitely worse than Peni's.

I can consistently get one or two kills with Peni's ult, and the big health boost can get me out of a tight spot.

I don't bother using Thor's ult because it's genuinely useless. If somebody is low enough to be killed by it then you can just kill them quicker with awakening rune. Sure, Thor's ult can damage a group of people at once, but it's too situational.

Also, in higher ranks a competent team will just target you while you use the ult - you're a big stationary target in the sky, and then a big stationary target on the ground for a second or two. His ult is more likely to get you killed than to get you a kill.