Yes, Realtors are people, too. Their opinions are based on dealing with hundreds of clients and customers who actually express opinions about the school districts and schools. If a Realtor expresses an opinion about schools, you should trust that over some blogger or random website that comes up on Google.
It makes little sense to believe someone who says "don't trust Realtors" from one side of his mouth and "call me, I'm a Realtor" from the other side.
You can also go to the state's website http://www.tea.state.tx.us to see what officials think of the various school districts and how they compare to each other. Of the two districts you mention, Highland Park is without a shadow of a doubt, the better.
Richardson has very good schools too but it has a larger population too. There are 53 schools in Richardson of which:
27 are exemplary (all elementary)
19 are recognized
7 are academically acceptable
Also note that school districts in the DFW area overlap cities, so for example, you can live in the city of Garland but be zoned for the Richardson ISD... the same all over.
Let me know if I can help.
OTHER OPTIONS PER ARTICLES AUTHORED ARE:
Coppell, Colleyville, Southlake, Trophy Club, Highland Village, University Park (which many homes located in HISD), Plano, Frisco, McKinney.
Determine on your budget, personal/professional lifestyle
Plano is also a top city to live per many studies.