ttonk,
We have both trees now for 5 years. The AB will grow MUCH faster, though the FF grows very nicely for Sugar Maple. The AB is much more soil tolerant than the FF. However if you have suitable soil, the FF has given us no issue. The Sugar Maple is not drought tolerant either, though again, relative to Sugar Maple it does well. Lastly, FF is NOT predominately yellow in fall color. In 2011 ours was a Yellow-Orange and Orange with some Orange-Red, in 2012 it was mostly Red-Orange and Orange. Other years it has been Orange. In our climate it is somewhat variable, but it has never been yellow. For other trees, go post in the "Tree Forum" (which is where I am usually), where you will get variety in ideals.

Arktrees

This post was edited by arktrees on Sat, Jan 26, 13 at 0:47

I agree with Arktrees. Autumn Blaze will definitely grow faster and be more tolerant of less than optimal soil conditions. I am all for diversity in trees, but as these are both Acer, any disease or insect will probably impact both, so no benefit there. Consider other seasons too, If one tree can be much bigger then the other and your yard will still look good, go for it. If it will look odd, keep the two AB's.

Here is a link that might be useful: When to move plants

Thank you both for kind answers.

Which cultivar of sugar maple that is comparable to FF or AB in terms of other good qualities would have consistently yellow fall color?

Would it be better if I plant a larger FF? ABs I planted are about 1.5 caliper, 8ft tall. A bit sad looking with their thin, upward-pointing, & asymmetric branches. I don't think I can expect anything spectacular this or next year. Guess it will take at least 5 years to have a good shape and shade? If I plant a sugar maple that is 15 ft tall, then they will balance out, I imagine. But it would be quite costly given that I've already spent some for AB (especially the "planting" part), and large trees are expensive, and they do not warrant larger trees. Maybe I should give it a chance.

By the way, I have a clay soils. I believe it is basic soil, not acidic.

This post was edited by ttonk on Sat, Jan 26, 13 at 2:48

What did you end up doing ttonk? :) I was going to tell you Green Mountain Sugar Maple. But saw this is from 2013.