particle が - が vs を in sentences of desire (-たい)

According to Genki, expressions of desire ( -たい sentences such as in the examples below) which use the particle を can also use the particle が interchangeably, and besides stating that, they give no further explanation. From what I could tell from searching around it seems that が appears to be a more natural choice (I could be wrong though). Does using を vs が imply a change in meaning? Is one particle preferred over the other in these types of sentences? Can they be used interchangeably 100% of the time or are there exceptions?

e.g. 映画が見たいです and 映画を見たいです, both of which should mean "I want to see a movie".