While I'm not nearly as technically gifted as some of the others, from I've read, Spigot seems like our best bet, and heres why I think so.
Its been around definitely has a lot more experience than Sponge. Sponge is entirely new and if I'm understanding correctly, doesn't have a lot of the features we're familiar with with Craftbukkit yet. In Super's link to the Sponge post above, theres a section titled "Rolling Releases" which says:
After several rolling releases(we project up to v1.5 or further) the server API will be declared stable in its entirety. Plugin developers can expect these features to change or be added before then:
Events
Inventory
NBT
Permissions
Simple database support
Now, I may be an idiot and not understanding this correctly, but from this I'm gathering that Sponge currently does not have any of these features.
Spigot I believe already has the "goodies." It's based on Craftbukkit, which is what all of us, dev and director alike, are most familiar with. Theres already PEX and WorldGuard made specifcally for Spigot. People are familiar with the API and its not an entirely new one like Sponge is.
While someday I think we can look at Sponge as an option again, I think it would be best to stick to what we know and wait to see what direction Sponge goes in.