Hrmm.. I dont' think that either of those reasons had anything to do with it... 1) becuase Sirius befriended Lupin, just like James and Peter:evil: even though they knew he was a werewolf, and werew good friend iwth is for a long time, so i think that they would have gotten over the stupid predujuce, just like Harry Ron and Hermoine have! and 2) Sirius didn't know about Quirell.. he was put into jail 10/11(?)yrs before that happened... and there has been no hint that any of the DADA teachers went evil before Quirell...
I think that the reason that he suspected Lupin was the same reasont hat Lupin suspected him... they knew that someone close to the Potters was slipping Voldie info about them... who was closer that those two... Also, if people had suspected Peter, then it would have been more likely to guess the end, and the fact that he was really weak/not as talented sorta made tehm all think that it wasnt' him...
That's just my opinion tho...
