I have both S2 and SP10 in my racks...

SP10 is second to none in build quality and reliability.

S2 was built to be a cheap box they can just toss at customers and lower the price as needed -- like the S1.    

My S2 arrived as a pile of parts, i had to reassmble it and 4 of the hash boards were damaged in shipping so its hash as about 600 GH/s.  Trying to get replacements but communication is difficult.

SP10's are super high quality, very solid performers and a bit better power efficiency.    The web interface is very straight forward and simple to configure.

@ $3200 SP10 group buy VS $2599 for the S2, i would (and did) pay the extra money and buy the SP10 all day long.   Both should ship within a week or two.