I know that in House's ending, if you make Arcade fight during the battle for Hoover Dam, he gets hunted down by rangers/securitrons.

What happens to him in the Indy ending if he fights?

I think he disapears into the wasteland and is never seen again.

Or he settles down somewhere, I forget.

It has been a while, but I'm pretty sure it just says that while an Independent Vegas was stable, it wasn't as good as Arcade expected it to be.

Then he disappears into the wasteland and is never heard from again (I think).

Best ending for Arcade is to side with the NCR and don't involve the Enclave in the battle. In that ending, he'll stay onboard and the NCR won't do it's witch-hunt, thinking that the Enclave are permanently dead.

How is that better than the House or the Indie ones where you do the same?

It's pretty much the same ending for him, he hides that he's Enclave and people regard them as history.