both are weeds in the right conditions. UM has a nasty tendency to grow bushy and then rot at the roots when it gets shaded, the whole mass then floats up and is impossible to replant as its so bouyant. finally gave up after 3 10x12" bushes floated/pulled up and gave it away. strangely, my e. triandra does the same thing and grows in massive bush form till it pulls up and floats.... going rip up my HM this weekend, as its formed a solid wall in one corner of my 120g and looks more like a box with leaves on it than a bush plant. its rotting away at the base also due to lack of light getting to the root portions. gonna try some rotala ceylon, rotala vietnam and diplis diandra and see if that does any better (with a little more diligent triming of course).
I have a bush of HM in my tank, it started as a few strands that came with some fish, and months later it is the size of a cantalope. I keep cutting all of the runners back and give it a trim every once in a while down to a softball size. I think I will try and sculpt it into a shape like a triangle or something.