It's tough to compare the two as each is a different type of PG. However, Nash is the better pure PG and Parker is probably a slightly more complete player.

It's also tough to compare the two because they are asked to do different things by their teams. Parker is a second banana whose life is made easier by Duncan. Nash is unquestionably the Suns' leader and their most important player. As such, I think Nash's flaws are more easily visible because he has and will continue to be judged on whether or not he can take the Suns to the promised land as a franchise player. Parker, on the other hand, need only be very good to have been judged to have done his job.