what stripey said, basically
a spinal will be used for planned sections or emergecny sections where no epidural has been sited alreayd and there is time to give a spinal, rather than a general
epidurals are used in established labour and can also be recommended for women who have high BP as they do lower your BP
with an epidural , there are pros and cons for labour... for me, the biggest con is that you are rendered pretty much immobile and even with a mobile epi, you are not really able to get up and walk . and being active and upright is really hlepful
the biggest pro is that if your labour is long, the epi can help you to relax or even sleep to regain energy and then you don't have to have it topped up ,so that you can feel when to push
it's worth reading up about them, they do take your labour into a different arena, all otehr pain rlief options can be given by a MW, but an epi requires an anesthatist, you will also have to have a drip, so your mobility is going to be restricted