Are you on PS3 or 360? If on PS3, I know both are in the demo downloads on the Playstation Store. I don't know anything about the 360 network demo availability.
I tried both games and I did not particularly like either one(but keep in mind I'm ridiculously picky and only about 1% of available games ever interest me enough to play them more than an hour), but the graphics on the models in UFC are superb, that's for sure.... but the ring floor texture is rough looking on the PS3 version; the 360 has higher res texture on the ring floor graphics; other than that, I did not see a difference on the models between the platforms. Fight Night's graphics are also excellent, but the models don't look very much like the people they are supposed to represent, unlike the UFC game, which did a remarkable job with likenesses. I suspect UFC actually used body/face scans of the characters, where as Fight Night just had the model makers 'fudge' likeness by eye.
-Chris