Assad, while an asshole, is involved in global politics. ISIS is not; it isn't considered a country, like Assad and Syria. Assad isn't going to go to war with other countries in fear of international retaliation. His higher stature controls him to some extent, just like North Korea. ISIS, on the other hand, because of its guerrilla style nature and less formal structure and association to other countries is more volatile and more dangerous. I think Putin is choosing the lesser of two evils.