Apologies for a late response, but my 2 cents.

Well at this age both OSPF and ISIS are equally usable as an iGP for a service provider as both have equally matured from a vendors's development standpoint. 

While ISIS is widely used by most ISPs, there is a known ISP in SA that has always used OSPF comfortably for years. But the main reason most operators stick to ISIS is its historical stability and development by the vendors back then. It is therefore assumed that in the event of feature enhancement, a vendor will most likely apply such on ISIS before OSPF due to expected high pressure from big ISPs. Otherwise both have same concept, but different terminologies. 

Regards,

Simon