Neither is completely accurate. For folks who never play in private games they both do a pretty good job, but for folks who do private games, the stats are misleading. Neither apparently can get the results from a private sng or tourney. Either that or they don't try.
In my case they will show I 've had a few bigger wins, but probably still show me as break even give or take a bunch. However, neither shows that over the past year my br (directly related to my ITM's) has grown, making me an above break even player. That is because of the private games I sometimes do well in.
Both SS, and OPR will give you more info than you had before you inquire, but you need to take that info cautiously.