They will root in the substrate, or they will grow floating. It doesn't matter much to them. One problem with water sprite is that it can get too big for almost any tank. Finally, you will have to tear out the big plant and plant some of the smaller plants that have grown from the older leaves of the big plant.
I've had plants growing from the same original water sprite now for 15+ years. Definately do better if just the roots are anchored in the substrate. Too deep and they rot. Little bit doesn't seem to matter.
The floaters multiply the fastest. The planted ones get bigger. It's kind of a neat plant. If you have an open topped tank it will grow out into the air too.
Ceratopteris is more of a crown plant and does not have a rhizome to speak of. I agree, the crown should not be buried. There are about four different varieties that have been seen in the aquarium plant trade. One of the varieties, the "oak leaved" Ceratopteris, is very scarce. I have a picture of two of the three commonly available varieties in the album under misc. photos. http://www.aquaticplantcentral.com/forum/album_page.php?pic_id=143