I say bring on the Pens. Because:
1-I won't be as pissed if the Sens lose against a stacked team
2-I think the Sens actually matchup better against the Pens than against the Bruins. Bruins have better goaltending and stifling defensive play, that one wouldn't be pretty. On the other hand, the Sens might be able to do exactly that to the Pens. If Karlsson is healthy, I could see him having just as big an impact as Crosby on the series.