I've seen both, and Log Horizon is definitely better so far.
SAO gets extremely bland by the time the seventh episode comes up. It's like the whole first arc is nothing but filler, aside from the first episode itself. I don't even know why I watched the whole show.
Log Horizon, however, seems like it'll be a bit more like Shingeki no Kyojin coming soon. However, I've never read its novel.
My one objection with Log Horizon is that people can't die. It takes a lot of the tension out of the anime. Log Horizon, however, makes this OK, given its premise that they're stuck potentially forever.