I've had 'em both, & prefer Yunnan Gold (to the point where I've consumed the sample and 1.5+ oz. in the past month or so.
The Gold has an interesting complexity, a sweetness, a depth, malty overtones & tends not to go bitter when I get distracted.
I found the Assam to be just okay -- not bad, but uninteresting. And could get bitter if not decanted after 4 min. Besides the Yunnan Gold, I preferred Golden Monkey, Darjeeling 2 and 22, & Ceylon Sonata. Also Oolong 40 which, tastewise, strikes me as a black/oolong hybrid -- it's the tea I drink most often.
And I'd second Chip's comment -- what you want to drink at a given time will vary with your mood.