Just finished watching SNS2 and SNSS.
SNS2 was mediocre UNTIL the final 4 episodes, where it got INTENSE. There were some real plot twists at that point, and a lot of new spells/abilities were used, it was really really good at that point. However, 20 mediocre episodes can't be ignored, getting up to those final 4 episodes was a pain in the ***, and just disgustingly boring. SNS2 gets about a 6/10 from me.
SNSS is the OVA series, it's got a really nice feel to it. It focuses mostly on the relationships between Yuji and Carmel-san so it really makes a refreshing change. It gets about a 7/10 from me, purely because whilst the interesting set ups for this are alright, it really lacks any action packed scenes, but it's definatly one of the better OVAs.