I fancy Middlesbrough at The Riverside Stadium however I don't see them repeating the scale of their heroics in the previous round. This (IMHO) is going to be a gritty match, very hard fought, similar I think to Arsenal's Champions League match at Villarreal's Estadio El Madrigal. I see this going into extra time or maybe even penalties.
As for the other match, it's going to be a hard job for Schalke 04, especially as they'll be playing away, but I see them pipping FC Sevilla 0-1, 1-2, or possibly even qualifying for the finals on the away goals rule.
Respectfully then my money's on Middlesbrough and Schalke 04...which (knowing my luck ) could mean the "the kiss of death" to both these clubs!
I agree on M'boro, but I strongly believe that Sevilla will qualify...they could rest in the weekend, as their game was postponed, and Schalke is in very very bad form in the german league...and even the best Schalke would have a 50-50 chance against Sevilla imho....