I'll take a small stab at this and hope for reinforcements. I have one tree of each and need to take a closer look at them this weekend. To keep them straight I have a map listing what I planted and where, but then I also planted them with the Sungold in the East (where the sun rises and the moon is in the West since you see the moon after the sun has set--sort of).

My Moongold has a more upright and growthier habit than the Sungold. It also tends to bear all along the elongated upright branches on year old wood, and has a slightly larger apricot. Both trees are 3 years old at this site and this is the second year for fruit. Not many this year compared to the number of flower buds and bloom, but they both made it through below 0 temperatures in mid-late April when the buds were expanding-though it probably took out 90% of the flowers. The Moongold reached about 10 feet tall this year before I headed back the most upright growing branches. The Sungold doesn't grow as vigorously and has a denser growth habit. It is about 6 feet tall before I did some thinning and slight heading pruning to work on an open vase shape. The fruits were more concentrated in the center of the tree rather than all along the branches. I think the Sungold fruit also have a better taste, although the other is by no means tasteless or bad.

They both bloom at about the same time, although I think the Moongold was slightly later and the buds came out over a longer period of time. The Moongold fruit also took a bit longer to ripen. Both trees grow spurs that look like giant thorns, but they were a little smaller and denser on the Sungold.

I'll get a good look at both trees over the weekend and see how much my memory is off!

A lot of these traits I've noticed could just be due to variations in location--variable soil could allow one to be more vigorous than the other, etc, and the result of the extreme cold in April may have affected the buds a little differently, so others might have very different results.

Took a look at the trees this weekend, but don't know that it helps much. The leaves on both are both about equally round and similar size--anywhere from 1-2" and varying somewhat across the tree.

Both have a lot of buds on the growth of smaller branches. The big growers this year that put on several feet don't show a lot of buds other than some at the ends. Also, as I think about it, the Sungold was the one that ripened later--about 10 days to 2 weeks later than the Moongold.

Both are starting to turn a bit. Haven't watered either since the end of August and had an inch of rain Labor Day and the week after. Try to dry the trees out going into fall to help encourage them to mature their new growth and go dormant, then give them a good soak as soon as they do go dormant so they aren't completely dry over the winter.

Can't help much I guess with telling the difference between the two--there are differences, but they seem quite subtle and I can't explain what they are other than they just look different.

Thanks!