With every passing day, the science behind the utility of umbilical cord blood is going a step forward.
It is increasingly being used for curing leukemia patients.
Umbilical cord blood is rich in stem cells.
As the name suggests, it is extracted from the umbilical cord and the placenta immediately after delivery.
Stems cells are very important.
They have the capability of creating entirely new blood and organs too.
The cord blood stem cells have created your baby and by preserving those stem cells, you are giving your baby a greater chance to fight possible life-threatening diseases.
Cord blood Vs Bone Marrow Transplants For Cord Blood:

Research shows that the quality of the stem cells taken from the bone marrow is not as high as that of the umbilical cord blood.

Patients who have received a bone marrow transplant are more likely to reject the new addition, which could lead to life threatening situations.

It may take months for the medical team to find a donor with a matching blood composition.
With the patients cord blood available, the treatment is not delayed beyond a few days.

In case the patients cord blood is not available, public cord blood banks have a vast resource of stem cells, which were donated by new parents.
The cord blood banking system is highly computerized and one can search for the matching composition very quickly.
For Bone Marrow:

Cord blood transplant is a relatively new compared to bone marrow transplants

The quantity of umbilical cord blood is limited.
The amount of stem cells is very crucial for any treatment.
A younger patient will require less stem cells compared to an older one.
If for any reason the patient requires a second graft, he will have to take a donation from outside.
This may lead to more complications.

Stem cells are slow.
They take time to produce the required blood cells.
Till then the patient runs severe risk of infections due to reduced immunity.

There may be case where the stem cells are not be as healthy as expected after being preserved for so many years.
Theoretically, cord blood can be preserved forever.
In reality, no one is sure about how long a unit of cord blood can be saved without affecting its utility.

At the end of the day, it is your decision.
Nevertheless, do remember that even if the option of bone marrow transplant is available, your child will have a greater chance in emergencies or critical illnesses when there is no matching donor nearby.
It is not necessary that your child will have a sibling with matching genetics close to him all time in the future.
The cord blood, which you had saved years ago after his birth, will be enough for him to start life afresh.