Hi charli'smumma
There are a few differences between a spinal and an epidural which I will list for you.
Spinal
Needle can not be left in place so basically it is a one dose only anesthesia.
Anesthetic is injected into the spinal fluid. (the fluid that surrounds the spinal cord).
Usually produces a denser block thus preventing you from feeling anything including contractions.
Can have more severe side effects than an epidural.
Often used for emergency CS as it can be a little quicker
Epidural
Can be left in place so top up?s can be given.
Anesthetic is injected into a space just before the spinal fluid.
Block is not usually as dense as a spinal thus controlling the pain but letting you feel the contractions and possibly leaving you with enough control to push.
If you are going to have a CS then they will increase the amount of drugs or use a slightly different mix of drugs to give a dense block.

There would be no different with inserting the needle as for spinal or epi they need to pass the needle pass the bones of your back. The gap between the bones is quite small and it can sometimes take a few goes to get the right spot.
It is not all that common but sometimes a spinal or an epi do not work. Other times they only work on part of your body.
A spinal is not done as often as an epidural and there is a chance that some doctors are more experienced with doing an epidural and prefer this to the spinal.
I find it a little strange that your doctor did not top up your epidural if only 1 leg went numb. Also he could have got you to change position. This will help to distribute the drug around your spine more and thus blocking more nerves. Of course if there was a need to get the baby out quickly then I can understand the need for a general.