The Doctor was designed to mimic sentience to a point. Data was designed to be sentient. The Doctor is however a lot larger than Data, the whole of between the walls in sick bay (initially) and 40 years more advanced than what Soong was working with. Data is a stunning example of miniaturization and mobility, whereas the Doctor is neither. The Doctor may be able to appear more sentient than Data because his existence is the product of millions of competing engineers perfecting the science to a degree of hyper realism for the last 20 years, but seeming more "real" isn't what makes you sentient, it's about how you think, how you come to decisions and what limits the proccess.
Of course by that reasoning a Borg drone isn't sentient.
It's a sleeping person.