UltraHD has a bit better color, wider angle lens, stereo mic. The sound from the UltraHD is much better than the MinoHD. Also, you can use AA batteries if your rechargeable runs out, or just carry other rechargeable packs with you.
Check these out and see what you think. This is what helped me make my decision.
http://vimeo.com/4405204?hd=1
http://vimeo.com/4405767
http://www.vimeo.com/4562481
I picked the UltraHD. I think the colors are better, sound is better, and the wider angle is nicer.
MinoHD is really nice, as well. The sound is great for its size and its competition, the video is nice and clear, the form factor is great, and its just slick for an HD player.
They both have semi poor low light performance. There is considerable noise with night shots.