Depends on class of service. In economy I would go with LH. That sfo-dxb in steerage would be brutal, and emirates economy is pretty bad. Emirates business class is a lot nicer than LH's, I have to say, and the long flight wouldn't be such a problem. In F it's a toss-up. Emirates is better inboard and LH better on the ground. Emirates ground services are ridiculously bad, especially considering how much effort they've expended on the in flight experience. And everybody has to try the FCT once in their light, even if it's just the experience of getting driven to the plane.

Either airport is fine for transit, neither is up to the standard of, say, Singapore. On average I'd say there's more of a "kettle" factor at DXB, but they should get some credit for the pretty cool flashmob they did.

One additional thought is that if there are any problems you're much better off with LH. They will reroute you on other carriers and there are lots more pssgr protections in the event of cancellations and such. The motto over at Emirates is "we've got your money already, what do we care happens after that" or, more saltily "the passenger can go %*#}%€£>|] himself".