Which one is better as written or as a story idea?
I personally like the Xindi arc better in both ways. It had a much more complex plot, which is a good thing. In DS9, they didn't have to find out why the Dominion was attacking them, they knew it from the start. In Enterprise, they didn't initially know why Earth was attacked and who were the Xindi. The Dominion war was just a war and there was nothing mysterious about it.
I personally like the Xindi arc better in both ways. It had a much more complex plot, which is a good thing. In DS9, they didn't have to find out why the Dominion was attacking them, they knew it from the start. In Enterprise, they didn't initially know why Earth was attacked and who were the Xindi. The Dominion war was just a war and there was nothing mysterious about it.