I have flown both, and have tons of photos somewhere from the Air China 747-8 flight but never got time to do a trip report.

the CA 747-8 Biz seats are indentical to the UA 787-9 seats.

If you can get upperdeck on the 747-8, that will be a plus.

IFE: UA is miles ahead
Services: CA is actually getting better
Wine: CA has a lavish wine menu and if the selections are available, pick CA over UA.
Food: CA over UA (I am tired of the UA selections)

I love the 747-8. Having just flown them recently on a few LH Transcontinental flights and I can definitely notice the engines are less noisy than 744. The redsigned interir has the 787 feel to it. The upper deck bins are now able to take on full sized rollers is a huge plus.

If you fly UA often, you will get other opporunities to fly the 787. 747-8 is more limited because there are only less than a handful of airlines flying them.

I have done a number of SQ and LH A380 flights, but, IMHO, 747-8 upperdeck is still a better experience.