I am a big fan of Hemianthus Micranthemoides.
Here is a picture of it growing in my tank:
Since then it has grow in even thicker with repeated trimmings. I absolutely love the plant for this purpose!
I have the four leaf clover in my tank, while it is a cute plant it is VERY unimpressive. The growth took off at first, but quickly slowed to a crawl. It does not spread as well as I had originally thought it would, but I will leave it in where it has grown so far.It took about 2-3 months of trimming and replanting from about 10 short stems until it started to look like a carpet.
The thing I like most about HM is that since each individual plant has lower light requirements, it can get really dense without experience "die off" because the lower layers aren't getting enough light.
I'll post an update picture of it soon, but it is much more dense that in that older picture and is green all the way to the bottom.
At 1.61 watts of T-5 lighting per gallon, I don't have a super low-light setup but my tank is a 26 show so it is pretty tall and we have grown HM in cube tanks with lower light.
I haven't ever grown the marsilea but I like the clover leaf shape.