Somatic Stem Cells (also Adult Stem Cells) are non-designated cells that are found in adult organisms, capable of matching and replicating to many different tissues. Embryonic Stem Cells are similar cells found in embryo organisms, and are more prevalent and considered more potent. The difference comes both from the source of each and from their ability to match and replicate when used for therapy.
At this time, somatic stem cells are considered viable for therapy, and do not have the stigma of abortion attached to them. The harvesting mechanism is different as well; somatic stem cells are harvested from an adult without harm; embryos grown in a lab (similar to chicken eggs in that they have not developed past the viable level) are donated instead of being discarded, and stem cells harvested from them (this is considered the same as abortion if you assume life begins at conception). The moral issue is that both abortion and discard of unused embryos are both legal in the U.S., and so it makes sense to use them for research instead of simply throwing them away.
Another issue is that somatic stem cells are not quite as flexible in matching different tissues. Research has indicated that they should be able to match to the same degree as embryonic, but the science is not quite there yet. Additionally, it is harder to grow somatic stem cells in culture, while embryonic stem cells are fast and very potent. However, embryonic stem cells carry the risk of rejection, since they are not from the receiving organism; if somatic stem cells could be harvested, replicated, and reinserted into the receiving organism, this problem would be solved.
Until such times as either (a) abortion/discard of embryos becomes illegal, (b) somatic stem cells can be grown in large quantities, or (c) it becomes possible to grow embryonic stem cells without any fertilization process at all, there is no reason to avoid using the more powerful embryonic stem cells. Research is continuing.
With the constant advancement of science and scientific techniques, somatic stem cells, otherwise known as adult stem cells, have been increasingly researched and pursued as a viable alternative to embryonic stem cell research. On the moral/ethical level, adult stem cell harvesting is universally acceptable in scientific and religious communities as a sound approach in contrast to embryonic stem cell harvesting which has been hotly debated due to the conflicting determinations of the personhood of the fertilized embryo or fetus.
While many in the scientific community have pushed for research using embryonic stem cells a strong contingent of both religious and scientific communities object to the destruction of a human embryo for the purposes of scientific research. Since the arrival of adult stem cell techniques in 2007 there has been increasing evidence to suggest that the use of somatic stem cells carries greater promise of success. Since 1998 when the first embryonic stem cells were isolated and cultured, the efforts of embryonic stem cell research have led to no cures in diseases and yielded far fewer results than adult stem cell research. As a result many biotech companies have pursued somatic (adult) stem cell research as the more promising field of study.
In terms of recent scientific results, the adult stem cell rather than the embryonic stem cell has been identified as the more stable and viable of the two for use in therapeutic treatments. Since the adult stem cells by nature work as a natural repair mechanisms in the human body, it is better suited for therapeutic treatments than embryonic stem cells which, due to their nature react better in a microenvironment of a rapidly growing embryo.
In terms of the current state of affairs for stem cell research adult stem cells have provided more immediate results but given time and more in depth study, additional methods such as Embryonic germ cell therapies (ethically sound when obtained by informed consent from miscarriages) may yield results.
In the field of biological science stem cell techniques are relatively new and while a number of specific advances have been made, more unexplored questions are currently being worked out in the field and will rapidly continue into the future.