I don't think it's even close -- Lion King is so much better.
One thing to keep in mind is that most people seem to either love or like Lion King, while there are people who dislike the Tarzan show. It's been a long time since we've seen the latter, but my musician dh thought the performance quality was truly bad. Where we were sitting (back -- righthand side), though, the sound quality was not what it should have been.
I've decided I should see it again some time, b/c that day was not our best ever at WDW. It was extremely hot, we had a bad lunch (at Flame Tree -- I probably also should give it another chance), and my child with Asperger's had a meltdown. We returned to the resort immediately after Tarzan.