I would get the SP on the terrain for your races, as it differs according to region from what I can gather. If you can get away with spikes for all or most of the races IMO they're a good investment cos they tend not to be too expensive, will last you a few seasons, and if conditions are really soft and muddy there's no substitute.
If you do decide to get spikes you MUST try a few different pairs to get the best fit. There's no worries about correct shoe for pronation, etc. You want a snug fit around the mid-foot and heel, so that they don't get sucked off your feet in the mud; fit can vary a lot between manufacturers, from very narrow (NB from my experience) to more roomy (Brooks, Nike, Asics...)