The main difference is the so called "cooling-off period" that serial killers go through. Committing a violent act provides both categories of killers with an emotional, psychological or sexual release (sometimes all three) and in the case of a serial killer this means that he can lead a seemingly normal life before he feels the need to kill again. Spree killers on the other hand escalate once they start killing, the period between murders is shorter and shorter and they devolve a lot faster.
Another very important difference is the fact that generally serial killers are more intelligent than spree killers.
There is no difference in motivation between them.