I am considering building a SAN unit on Server 2012R2 using Storage Space. In that situation is it more helpful to have more CPU cores, or less faster cores?
Pretty irrelevant - as in: you can go for a midrange CPU and get enough power these days for a SAN. Go for price (and power envelope as this implies cooling).
THAT SAID: Make sure you know what you want to do with the storage spaces. There stil are serious issues with performance when using a raid parity setup. If you need fast write in Raid volume you basically are out of luck. Resizing also is limited (no rebalancing of content). If you can live with these limitations... things are ok.