I don't think Sutil is a dirty driver at all.
Also the performance of drivers imo rely a lot on the current charateristics of the car
and atm Force India's (only) strength is, that they can make the tires last on an acceptable level for a very long time, so it would be stupid for them not to do these kind of strategies.
Also you can't forget that driving on the edge on older tires is generaly more risky, so it isn't that unusual that DiResta had some offs this season, since he is usually the guy always trying less pitstops (mainly because Sutil has the edge at qualyfing most times).
I think as soon as Force India gets a car together that is stronger on pure laptimes again, Sutil will be the faster of the two.